war to spread once it started, involving many countries rather than just two.

Imperialism was another cause of WWI.  Many European countries already had empires before 1900.  France and Britain, in particular, had large empires. Germany wanted to be seen as a major power so it wanted an empire as well.  As it tried to take an empire, it worried France and Britain, making them want to fight to prevent Germany from encroaching on their empires.

Finally, nationalism was a major cause of WWI.  This caused countries to be arrogant and aggressive because they felt that they were superior to other countries.  It also helped to create the alliances and to cause the assassination that led to the war.  Slavs allied with other Slavs.  Germans allied with other Germans.  These alliances helped lead to WWI.

Thus, we can say that there were four MAIN factors that caused WWI.

There were several causes of World War I. Some had been building up for decades and just needed a spark to start a huge conflict. One cause of World War I was imperialism. Countries like Germany and Italy, that became unified countries around 1870, wanted to gain colonies. However, by the time they united into a country, most of the land available for colonization was already gone. Thus, one way to get colonies was by going to war.

Another cause was nationalism. People within various countries began to develop intense feelings of pride. They believed they could do whatever they wanted because their way of life was the best. They weren’t worried about what other countries might think or do because they felt they were better than any other country. If a war started, they believed they would win the war because of these strong feelings of pride.

Entangling alliances and militarism also led to World War I. Countries were building up their militaries throughout Europe. This was an ominous sign because usually there is a reason for a building up of the military. Since countries had formed alliances with other countries to protect themselves in the case of an attack, a situation developed where a conflict between a country in one alliance against a country in another alliance could easily escalate into a multi-nation war. This is what happened in World War I, when Austria-Hungary, a member of the Triple Alliance, declared war on Serbia, a country that was closely allied with Russia, which was a member of the Triple Entente.

The assassination of Franz Ferdinand was the spark that ignited World War I. There were people in Europe that were being ruled by people who weren’t of the same nationality as they were. For example, some Serbians were ruled by Austria-Hungary. These Serbians wanted Austria-Hungary to give the area where these Serbians lived to Serbia so that Serbians would rule Serbians. This concept of self-determination existed in many places in Europe. When Austria-Hungary refused to do this, a plot was developed and carried out to kill Franz Ferdinand, the next King of Austria-Hungary. This assassination set in motion a series of events that led to the start of World War I when Austria-Hungary declared war on Serbia, and Russia, a close ally of Serbia, declared war on Austria-Hungary. Other joined the war because of the alliances they had with each other. World War I had begun.

I think that most historians would agree that nationalism was a primary cause in World War I.  The nations involved in World War I were guided by a belief that their country was exceptional and worthy of dominant status.  This love of country was beyond patriotism and moved into a realm of nationalism in which one's dominance had to come at the cost of another nation.  Logically speaking, if multiple nations possess this mindset, armed conflict in the form of war becomes inevitable. Nations such as England and Germany were focused on a dual pronged approach in which fear of "the other" and asserting their own strength were complementary. This made conflict inevitable and helped to explain why it was initiated so quickly.  Accounts of the start of the war brought out this nationalism element as a cause for the conflict:

The first month of the war resembled a month-long patriotic festival. In the first three weeks of August, Germans said goodbye to their troops, smothering them with flowers and so much chocolate that the Red Cross asked the people to be less generous: the soldiers were getting sick.

The combination of nationalism and its vehicle of armed conflict played an important role in starting World War I.

Interestingly enough, imperialism and the reality of resources dwindling are another important cause to the war.  At the time of the war's outbreak, European nations had been on a type of race to colonize the world.  The expansion of imperialism all over the world created a "colonies race."  Yet, it also served to heighten tensions.  As more nations fell under European flags of control, there were less nations to colonize. The reality of scarcity began to impact decision making in European governments.  Different parts of European society began to see the emergence and control of foreign markets as leverage against aggression while protecting resources and means of wealth: "Public opinion in Germany was feasting on visions of Cairo, Baghdad, and Tehran, and the possibility of evading the British blockade through outlets to the Indian Ocean."  Allied forces like England and France also recognized that they enjoyed privileged status in their colonial exploits.  The reality of coveting finite resources helped to move nations towards war.  Nations realized that in defeating other nations, colonial jewels would also become fruits of aggressive and militaristic labor.  It is in this domain where the need to preserve dwindling resources in the form of imperialism played a very important role in starting World War I.

The pursuit of secret alliances in the hope of facilitating greater land acquisition represented another important cause in World War I.  Nations had locked themselves into secret treaties with other nations in the hope of advancing their own claims.  The result became that when one nation declared war on another, they were actually involving multiple nations into the conflict.  Since these alliances were brokered in secret, the scope of the war widened because nations were unaware of who was aligned with one another.  This helped to make the conflict on such a protracted scale, creating a wide swath of death and destruction.

These three causes are significant to the development of World War I. Certainly, with such a conflict so much can be seen as causation.  Yet, in these three causes, the reality of World War I became realized.  

There are many causes of the First World War. Let us look at four main ones:

• Militarism: For at least three decades before the war, the powers of Europe, especially Germany, France, and Great Britain, had undertaken a program of military development, especially a naval buildup. On the one hand, these large militaries were intended to serve as deterrents to rival powers. On the other, they reflected an attitude that a nation's honor was tied to its military power. This belief would repeatedly contribute to increased tension in the years and months ahead of World War One. 
• Imperialism: Part of the reason for the military buildup was that the European powers thought it necessary to maintain and expand their empires. Germany in particular was anxious to acquire an empire that might rival that of Great Britain, and this led to repeated crises and an overall atmosphere of aggression and competition.
• Ethnic tensions: This especially applied to the Austro-Hungarian Empire, where many different ethnic groups vied for increased autonomy and even independence. Serbia, an independent state in the Balkans, sought to foster independence among Serbian people in neighboring Bosnia, under the control of Austria-Hungary. Indeed, many Bosnian Serbs wished to join their homeland to a pan-Slavic state. It was this struggle that led to the assassination of Austrian Archduke Franz Ferdinand in the summer of 1914, the spark that started the war.
• Alliance systems: It was the alliance systems that dragged the entire continent, and ultimately the world, into war. These alliances had been built many years earlier, and had shifted to the point that Great Britain, France, and Russia were joined in what was known as the Triple Entente, and Germany, Austria-Hungary, and Italy formed the Triple Alliance. War between any of the powers threatened to drag everyone into war. This in fact occurred because many of these nations also had other alliances. Russia, for example, was allied with Serbia, and when Austria-Hungary went to war with that country in retaliation for the assassination, it meant war between the two powers. This, in turn, enmeshed Germany in a war with Russia. 

On June 28, 1914, Gavrilo Princip, a member of a nationalist Serbian group called Black Hand, assassinated Franz Ferdinand, the Archduke of Austria-Hungary. The assassination, which took place in Sarajevo, was politically motivated. At the time, many ethnic Serbs lived in the Austro-Hungarian territory of Bosnia. Many of these Serbs, including the members of Black Hand, hoped that Bosnia would be able to break away from Austria-Hungary and unite with the neighboring country of Serbia. The assassination lit the fuse for World War I to begin. Austria-Hungary created a list of demands to the nation of Serbia, and when these demands were not met by the Serbian government within a month, Austria-Hungary declared war on Serbia.

You may ask how the assassination of the Archduke of Austria-Hungary by a Serbian nationalist caused a global war the likes of which the world had never seen before. This can be answered by examining the four major causes that historians have found to be at the root of World War I. The four major cited causes of the war are militarism, alliances, imperialism, and nationalism. These four factors can be remembered through the acronym MAIN.

The first major factor of the war was militarism. In the decades leading up to World War I, many European powers began the buildup of their militaries. Countries began dedicating more of their financial resources toward the enlarging of their armed forces. Due to the increased funding of the military, military leaders also gained greater power within their national governments. The increased influence of military leaders on government was most notable in Russia and Germany.

As the major powers of Europe learned of the buildup in neighboring countries' militaries, they felt that they must keep up with their neighbors for security purposes. This was arguably most evident in the naval race between Britain and Germany beginning in the final years of the 19th Century. During the time, each navy created numerous battleships, and the Germans doubled the size of their naval fleet. With so much invested into military spending, and with the increased involvement of military leaders in government policy, many European nations were eager to utilize their militaries. By the time World War I rolled around, European nations were well armed and ready for war.

The second major factor to examine would be alliances. Europe, in the years leading up to World War I, had a network of secret alliances. Many countries in Europe had developed secret alliances promising to assist each other in the case of war. These interlocking alliances were responsible for causing a chain reaction that expanded the war far beyond the initial nations involved.

Serbia held an alliance with Russia, while Russia held alliances with Britain and France. This led to the involvement of Russia, Britain, and France (known as the Triple Entente) when war was declared on Serbia. Austria-Hungary maintained alliances with Germany and Italy; this tripartite would become known as the Central Powers. Additionally, Austria-Hungary had an alliance with Bulgaria, and Germany maintained an alliance with the Ottoman Empire. With the declaration of war on Serbia by Austria-Hungary, all of the above listed nations were also dragged into the conflict. These secret defense treaties led a conflict between two nations to develop into a much larger war.

Imperialism was also central to the causes of World War I. The European powers had developed great rivalries due to their own imperialist ambitions. The major nations of Europe each sought to establish overseas colonies to expand their own influence and wealth. These rivalries, when combined with continued imperialist goals, provided added incentive for European powers to engage in war.

Nationalism is the fourth factor that is critical to the outbreak of World War I. In the decades prior to the outbreak of World War I, nationalism was on the rise in Europe. Each nation felt a sense of superiority to the others. Increased pride, plus a sense of superiority, led many to believe that if war were to come, their nation would be victorious. We can also see an example of nationalism in the motivation for the assassination of Archduke Franz Ferdinand. Black Hand had hoped to unite Serbs in a Serbian nation free from Austro-Hungarian control.

To summarize, it was a combination of four major factors that led to World War I. Military buildup made large-scale war possible and encouraged nations to fight. Secret alliances expanded the conflict from a regional one to a global one. Imperialist attitudes fueled European rivalries and provided added incentive for war. Nationalism led to overconfidence and a sense of superiority in Europe's great powers. With all of these factors building in the decades leading to World War I, all that was needed was a spark to ignite the conflict. That spark came with Gavrilo Princip's assassination of Franz Ferdinand. The result would be the deadliest conflict the world had ever seen to that point.

World War I would rage for over four years before the Treaty of Versailles would bring the war to an end with a victory for the Triple Entente over the Central Powers. The effects of World War I would, however, be felt for years to come. Not only would millions die and millions more be affected by the conflict, but the harsh punishment of Germany in the Treaty of Versailles would lead to the rise of the Nazi party, and ultimately to World War II.

The primary underlying cause of World War I was a series of alliances between European nations and a rising sense of nationalism by people of Europe who were denied a homeland. Poland had ceased to exist after it had been partitioned three times by European powers. Similarly, the provinces of Alsace-Lorraine (typically French) had been taken by Germany in the Franco-Prussian War. This latter event had been particularly bitter for the French who never forgave Germany for the insult.

Otto von Bismarck, in an attempt to protect Germany's position, had created a series of alliances which resulted in the creation of still other alliances in opposition. Ultimately two great alliances were formed:

• Austria, Germany and Italy formed the Triple Alliance.
• Great Britain, France, and Russia formed the Triple Entente.

Actual fighting began with the assassination of Archduke Ferdinand of Austria and his wife Sophie by a Serbian nationalist in Bosnia. Austria thereafter declared war on Serbia in response to which Russia began a general mobilization. Germany, fearful of a large Russian army on its border, issued an ultimatum to Russia to cease mobilization, which was ignored. Germany then declared war on Russia. Germany then demanded that France state its intentions, and France stated it would look to its own interests. Germany then declared war on France. To invade France, Germany troops invaded Belgium, whose neutrality had been guaranteed by the British. In response to the violation of Belgian neutrality, Britain declared war on Germany, and the war was fully engaged.

Austria-Hungary, a dual empire, had gained the right to "occupy and administer" Bosnia and Herzegovina, a fact which was bitterly resented by the people there who were culturally close to Russia.

There can be many different explanations for the cause of the First World War.  The immediate cause was the assassination of the Archduke Francis Ferdinand of Austria.  This plunged European nations into the conflict that the United States entered a couple of years later.  Yet, I think that such an event and the response that followed is the result of intertwining and concurrent realities that end up causing the conflict.  The growth and emergence of nationalism and the expansion of borders to a point where love of one's nation became synonymous with the overtaking of another is one such element.  Another would be the rise of militarism in European nations.  The military in different European nations were beginning to occupy an important role in the affairs of the country, making aggressivism a part of this relationship with other nations.  Secret alliances with other nations helped to plunge the entire continent into conflict as one member of the alliance was threatened.  In the end, there were multiple causes to the conflict.

The best way to remember the causes of World War I is by using the acronym MANIA: Militarism, Alliances, Nationalism, Imperialism, and Assassination of Archduke Franz Ferdinand.

Let's start with Militarism, or the arms race. Militarism is the glorification of the military. War seemed like a certainty during the years of the early 1900s, so Germany decided to increase their naval strength to rival of Britain's. Since Britain had the strongest navy at the time and wasn't about to let Germany rival their strength, Britain began increasing its navy. This, along with the rivalry on land between Germany, Russia, and France, led to much hostility between many European nations.

Alliances were also a major cause of World War I. In 1882, as war was looking more and more like a certainty, two major alliances formed. The Triple Alliance consisted of Germany, Austria-Hungary, and Italy (Italy would not fight with Germany and Austria-Hungary however) because Germany wanted to protect itself against France taking revenge for its losses in the Franco-Prussian War. When war broke out in 1914, Germany and Austria-Hungary made up the Central Powers. On the other side, the Triple Entente was formed by France, Russia, and Great Britain. Alliances allowed leaders to act more boldy than they would if they were alone. They knew with these alliances that they had powerful allies backing them.

Nationalism is extreme pride in one's country. Nationalism was strong in France and Germany. in 1871, when Germany was expanding its empire, France was defeated by Germany and lost the iron-rich territory of Alsace-Lorraine. France wanted it back since they felt it was rightfully theirs. This led to much hostility for years between Germany and France. Pan-Slavism also emerged. This was the idea that all Slavic people shared a nationality, so Russia, being the largest Slavic country, stood ready to defend the new Slavic nation of Serbia. Austria-Hungary and Ottoman Turkey were threatened by Serbia because they were worried Serbia would try to take some of their land to unite the Slavic people in the south.

Imperialism is when a stronger nation takes over a weaker nation, usually for economic reasons. In 1905 and 1911, a war between Germany and France almost started over Morocco, which was controlled by France. The more territory a country controlled, then the more powerful that country appeared to the rest of the world, especially economically. This also ties back into militarism as these countries that controlled overseas empires needed to strengthen their military to keep control of them.

The assassination of Archduke Franz Ferdinand on June 28, 1914 was the spark that led to the start of World War I. Ferdinand was the heir to the throne of Austria-Hungary, and on this particular day, he went to Sarajevo in Bosnia. Some Bosnians were Serbs who believed that Bosnia belonged to Serbia. A man by the name of Gavrilo Princip, a Serb, shot and killed Ferdinand after Ferdinand's driver made a wrong turn. Because of the death of their heir to the throne, Austria-Hungary sent an ultimatum to Serbia. Since Serbia did not agree to all of its demands, Austria-Hungary declared war on Serbia. The alliance system then led to a chain reaction of countries declaring war on the allies of their enemies. This was the start of World War I.

There were many factors that led to the start of World War I. One of them was nationalism.  A wave of nationalism was sweeping through Europe in the early 1900s. Countries began to believe they were better than any other country. They believed they could do whatever they wanted, and they could beat any country in a war if that country tried to stop them from achieving their goals.

Another cause was imperialism. Germany got into the colonizing mode very late. By the time Germany was unified, almost all lands available for colonization were already gone. The only way Germany could get colonies was to attack countries that had them.

A third reason why World War I began was militarism. Countries began to build up their military strength. Usually when a country increases their military, they usually have plans to use it. As Germany built up its military, so did the other Allied countries.

Entangling alliances were another cause of the war. When countries formed these alliances, it created the possibility that a war between two countries could quickly escalate into a multi-nation war. When Austria-Hungary declared war on Serbia, it led to a chain reaction where members of the Triple Alliance and members of the Triple Entente declared war on each other.

The event that sparked the start of World War I was the assassination of Franz Ferdinand. Franz Ferdinand was supposed to be the next King of Austria-Hungary. When a group of Serbian nationals assassinated Franz Ferdinand, Austria-Hungary made a series of demands on Serbia. When Serbia couldn’t meet all of these demands, Austria-Hungary declared war on Serbia. Russia then declared war on Austria-Hungary. This started the chain reaction of alliance members joining the war as one member of the Triple Alliance and one member of the Triple Entente declared war on each other.

Many factors led to the start of World War I. Most causes had been simmering for years while one cause was the spark that ignited the war.

Like all wars, there were many very complex causes of World War I, many of which had their origins in the nineteenth century. But the immediate cause of the war was the assassination of the Archduke Franz Ferdinand, the heir to the Austro-Hungarian Empire, on June 28, 1914. Ferdinand's assassin was Gavrilo Princip, a member of a radical Serbian group known as the Black Hand, who sought to drive the Austro-Hungarian Empire out of Bosnia and make it part of a pan-Slavic state under the control of Serbia. Princip and his co-conspirators had some support among the Serbian government, and the Austrian government used the incident to punish the Serbs, who the increasingly regarded as a threat in the region. With the encouragement of Germany, they issued a deliberately provocative ultimatum to Serbia. Serbian leaders turned to their ally Russia, who opted to mobilize their armed forces against Austria. Germany responded by declaring war on Russia, and the alliance systems dragged the entire continent into war.

There will be many different answers here.  I tend to think that one of the primary causes of World War I was the rising tide of nationalism.  Nations in Europe had started to embrace nationalism to such an extent that they sought to crowd out other nations.  Nationalism in World War I, or zealous love of country, represented a form of exceptionalism, or belief that one's nation is fundamentally superior to all others.  This made the embrace of war a natural consequence.  Nationalism was a critical reason why when Archduke Francis Ferdinand was killed, so many nations plunged into war so readily.  

Along these lines, I would say that another cause of World War I was the rapid rise of national militarism.  Advancements in war-fighting weapons made the war something easily envisioned.  The use of "new" weapons such as tanks, machine guns, planes, and chemical warfare enabled the fighting machines to become an accepted reality across the continent.  This helped to accelerate war with rapid intensity.  

At the same time, I would also submit that the presence of secret alliances being brokered so easily across the continent helped to increase the likelihood of war.  When nations entered into secret agreements and helped to develop the alliance system, when one nation was under attack, it pulled multiple other nations with it.  This helped to make World War I a protracted affair across the continent.

There were several causes of World War I. The immediate cause was the assassination of Archduke Franz Ferdinand of Austria. This led to a chain of events in which Austria, backed by Germany, held Serbia accountable for the assassination. When Russia, Serbia's ally, mobilized its troops in response, Germany declared war, and World War I had begun. 

The long term causes are more complex, and include:

• militarization of Germany, France, and Great Britain. Germany's crash program to develop a navy to rival Britain's was a real point of tension between the two, and both France and Germany had spent vast sums to develop massive, modern armed forces.
• nationalism in the Balkans. This was what led to the assassination of the Archduke in the first place. Serbia had designs on creating a pan-Slavic state that were at odds with the Austro-Hungarian Empire. 
• alliance systems. It was the alliance systems that transformed what should have been a regional conflict in the Balkans to a pan-European conflict.

These are just a few causes of what was a very complex series of events.