major insult and a hostile action by Germany.

Second, there was the issue of unrestricted submarine warfare.  Germany was trying to strangle the UK by sinking ships bringing supplies to that island nation.  Germany felt that this gave it its best chance to win the war.  Early in the war, Germany had sunk both merchant and passenger ships without warning.  To avoid conflict with the US, however, Germany pledged to avoid sinking passenger ships and to warn merchant ships so their crews could evacuate before being sunk.  By 1917, Germany had decided that these restrictions were making it impossible to sufficiently curtail shipping to the UK.  Therefore, Germany announced on January 31, 1917 that it was going to resume unrestricted submarine warfare. 

Although these are the most commonly cited causes of America’s entry into WWI, we must note that Wilson did not ask Congress to declare war until April 2, 1917.  This makes it less easy to say that either Germany’s resumption of unrestricted submarine warfare or the revelation of the Zimmermann Telegram was the cause of American involvement.

Once the US became involved, its entry was fairly decisive.  The Allies and the Central Powers had fought to a stalemate.  When the US entered the war, it brought in huge numbers of soldiers who could eventually be thrown into the battles and tremendous amounts of material support for the Allies.  This tilted the balance towards the Allies and the Central Powers were forced to surrender a few months after American troops actually started fighting in Europe.

There were many reasons why the United States joined World War I. The first set of reasons deals with actions of our enemies, mainly Germany, towards us. The Germans were violating our rights to trade with other countries. According to the agreed upon rules of war, a neutral country can trade with countries at war as long as they aren’t trading war materials. If a country at war wants to stop a ship from a neutral country and search it, it may do that. It can’t sink a ship without warning. When Germany began sinking our ships without warning after promising not to do this, President Wilson eventually went to Congress to ask for a declaration of war. Germany also tried to get Mexico to attack us. When the Zimmerman note was made public, Americans were furious at the Germans. Germany believed Mexico might want to help them in exchange for getting back some of the land Mexico lost in the Mexican-American war.

Another set of reasons why we went to war was idealistic in nature. For example, we wanted to make this war the last war ever. We also wanted to make the world safe for democratic governments. Unfortunately, these goals were not fully achieved over the long run.

The most common explanation for the US entry into WWI emphasizes that the US entered the war because of Germany’s policy of unlimited submarine warfare.  The US disapproved of Germany’s submarine warfare because it seemed cruel and because it was contrary to international laws that were supposed to guarantee freedom of the seas.  The US had been outraged, for example, when a German submarine sank the passenger liner Lusitania with over 1,000 civilians (among who were more than 100 Americans) onboard.  The Germans stopped engaging in this sort of tactic for a while in response to American criticism.

When Germany resumed unrestricted submarine warfare in 1917, US public opinion turned more strongly against the Germans.  This hostility was exacerbated by the revelation of the Zimmermann Telegram, in which the Germans tried to get Mexico to enter the war on the German side in exchange for which the Germans would defeat the US and give Mexico back the land the US had taken from it in the Mexican-American war.  This outraged Americans who felt that it was wrong for Germany to try to incite the US’s neighbor to go to war and who felt that it was wrong for Germany to promise to take away US territory.

Historians, then, tend to say that the US entered WWI because of its opposition to unrestricted submarine warfare and because of the Zimmermann Telegram.

There were a number of reasons for America's entry into World War I—moral, economic, and political. On the economic front, big business in the United States had a large financial stake in an Allied victory. Most of the Allied powers were heavily in debt to American banks and other lenders. If the Allies lost the war, then they would not be able to pay back their loans. By entering the war on the side of the Allies, then, the United States actively ensured the protection of American business interests in Europe.

Anti-German sentiment had been building in the United States for a number of years. However, what really captured the public's imagination more than anything else was the sinking of the Lusitania. Although the British passenger ship was likely carrying military equipment in addition to its human cargo, a large number of civilians—and some Americans—were killed when the ship was torpedoed by a German U-boat.

A further reason behind America's entry into the war was related to the two just mentioned. The Germans were operating an increasingly aggressive and unrestricted campaign of submarine warfare in the Atlantic. Despite promises to stop, the Germans continued to target civilian vessels. Among other things, the Germans' use of submarine warfare damaged United States trade with Europe, preventing American goods from reaching overseas markets.

The infamous Zimmermann telegram was another crucial factor. Zimmermann, the German Foreign Minister, sent an ill-advised telegram to the Mexican government, that suggested if the United States declared war on Germany, then Mexico in turn should declare war on the United States. In return for this, Mexico would receive territory lost to the United States, such as Texas and Arizona. Although the Mexican government had no intention of doing anything so foolish, the publication of the intercepted telegram inflamed American public opinion against the Germans and their allies.

When World War I broke out in Europe in 1914, Woodrow Wilson was eager to keep the U.S. out of the fighting. He ran for re-election in 1916 with the slogan "he keep us out of war," and his message had a great deal of popular support. A current of isolationism was sweeping the United States, in part because people felt that bankers were eager to enter the war merely to make money. People also saw the conflict as a European affair that did not involve the United States. Many Americans of German descent were not eager to fight against Germany, and many Americans of Irish descent did not support fighting alongside England.

What dragged the U.S. into the war was in part unrestricted German submarine warfare, including the attack on the British ship the Lusitania in 1915 that resulted in over 1,000 deaths, including those of Americans. After the Sussex, a French passenger ship, was torpedoed in 1916, the Germans pledged not to sink any more passenger ships but did not stick to their word. In addition, in 1917, the British intercepted the Zimmermann Telegram, sent from the German Foreign Office to Mexico proposing an alliance between Germany and Mexico if the U.S. entered the war against Germany. The revelation of this telegram helped sway American public opinion in favor of entering the war. Finally, the removal of Russia from the Allies fighting with Great Britain made entering the war more enticing for Americans, who did not want to fight alongside the troops of the Russian tsar. Wilson decided to declare war in April of 1917 with the idea of making the world "safe for democracy."

Before the U.S. entered the war, the Western Front had largely settled into a stalemate, though the British naval blockade of Germany was starting to have an effect on Germany. The Germans mounted an offensive along the Western Front in the spring of 1918, and Wilson sent troops to the Western Front to help the Allies. The addition of one million fresh American troops by August of 1918 turned the tide in the Allies' favor. The allies led a counteroffensive at Sommes that quickly turned the war towards the eventual victory of the Allies. The armistice was signed on November 11, 1918.

Originally, the U.S. proclaimed its neutrality in the conflict; although it was probably inevitable that the U.S. would eventually become involved. 

U.S. opinion settled firmly on the Allied side following the sinking of RMS Lusitania in 1915. Although Germany promised to no longer sink ships with its submarines without warning, and Woodrow Wilson commented that "there is such a thing as being to proud to fight," it was then only a matter of time. Part of the problem had been that the U.S. was sending supplies and other war materiel to the Allies which managed to avoid the German blockade of the British Isles. When Germany informed the U.S. that it would resume unrestricted submarine warfare (sinking ships without warning,) the U.S. broke diplomatic relations with Germany. Finally, when the British intercepted the infamous Zimmerman telegram, by which Germany sought to induce Mexico to come into the war against the U.S., President Wilson sent a war message to Congress. A formal declaration of war soon followed.

The U.S. was drawn into World War I for a multitude of reasons. Three of the most important are as follows;

1)   The Invasion of Belgium: At the beginning of the war, German decided that if they attacked France by going north through Belgium it would save them times and lives. The invasion didn’t go as smoothly as anticipated when the Belgium people began resisting, and soon rumors of German atrocities, many of them highly exaggerated, made their way to the U.S. and angered he public.  

2)   The Sinking of the Lusitania: Germany was having only marginal success stopping the flow of supplies to the Allied powers during the war. Ammunition and other war materials were being smuggled aboard American passenger ships, so when the German government changed it policy regarding submarine warfare, they began targeting passenger liners as well. One ship in particular, the Lusitania, was torpedoed in 1916 while carrying several American passengers onboard. The response in the U.S. was huge, and soon people began demanding that the government avenge the dead aboard the Lusitania.  

3)   The Zimmerman Telegram: In an effort to make the U.S. think twice about deploying a large force in Europe, the German foreign ministry sent a note to the Mexican government hinting that if Mexico were to declare war on the U.S. in the event the U.S. went to war, then Germany would support them. Mexico was still bitter from the loss of California and the rest of their northern territories, but the deal didn’t resonate with the Mexican government. Unfortunately for Germany, the U.S. intercepted it and the public outcry for war grew even louder.

To add to the above response, which covers it pretty well, in terms of trade, during the first nearly three years of war in which the United States was officially neutral, American trade with the allied nations increased steadily while trade with the Central Powers dropped off to almost nothing.  So while we were "neutral", Kaiser Wilhelm of Germany knew otherwise.

He also suspected that within many of these cargo ships, and even some of the passenger liners, were stored massive amounts of military supplies bound for her enemies.  It was the equivalent of someone saying they are not taking sides in a fight even as I see them hand my enemy a gun.  To him we were already in the war, and this led to unrestricted submarine warfare as official policy.  It is unclear whether the Lusitania was transporting war materiel or not.

The link below is a good summary of causes for US entry as well.

Originally, the United States hoped to stay out of the war; in fact many considered the Atlantic Ocean as a comfortable buffer to prevent that from happening. Literary Digest printed an article which read in part:

Our isolated position and freedom from entangling alliances inspire our press with cheering assurance that we are in no peril of being drawn into the European Quarrel.

A series of events over time led to U.S. entry, the first of which was the sinking of RMS Lusitania on May 7, 1915 by a German submarine which caused the death of 128 Americans. President Woodrow Wilson strongly protested this event, even though there is evidence that the Lusitania was carrying weapons and would thereby be a fair target. In an attempt to prevent U.S. entry into the War, the German government issued the Sussex Pledge:

Liners will not be sunk by our submarines without a warning and without the safety of the lives of non-combatants provided that the liners do not try to escape of offer resistance.

Events thereafter slowly moved the U.S. toward war. The British cut the cable between the U.S. and mainland Europe so that all news of the war reaching America was from Britain. President Wilson called for "Peace without victory" in a speech before Congress, but events had taken a different turn.

The German government, stung by the British blockade and unable to enforce its own blockade of the British isles, sent a diplomatic note to President Wilson on January 31, 1917 stating they would resume unrestricted submarine warfare (sinking ships without warning) the following day. On February 3, Wilson broke diplomatic relations with Germany.

Then, on February 25, 1917, the British delivered a decoded German telegram addressed to the German Embassy in Mexico City asking Mexico to join in the war against the U.S. in exchange for which Mexico would regain the "lost territory" which the U.S. had gained from Mexico in the Mexican War of 1848.

On March 17, 1917, German submarines sank three American ships. Wilson then asked Congress for a formal declaration of war on April 2; and war was declared on April 6, which ironically was "Good Friday."

When World War I broke out, the Wilson Administration's policy was to maintain a strict neutrality. This proved difficult to do, however, as most (but by no means all) Americans favored the Allied Powers, especially Great Britain. Moreover, the United States insisted on maintaining trade relations with all belligerent powers, and since Great Britain had imposed a blockade on German ports, this meant in effect that the United States only traded with the Allied Powers.

Germany responded by launching submarine attacks on Allied shipping, which often resulted in American losses of life and property, most famously when a U-boat torpedoed the Lusitania, a luxury liner that was surreptitiously carrying munitions, killing 128 American passengers. In response to US protests, the German government agreed to restrict its U-boats to military targets in early 1916. Yet one year later, facing starvation due to the British blockade, the German high command chose to resume unrestricted submarine warfare in a desperate bid to weaken the British economy.

This decision was accompanied by a (possibly forged) telegram to the German minister in Mexico, instructing him to offer terms for an alliance between Germany and that nation. These two events, combined with the sinking of a handful of American ships in March of 1917, caused Wilson to ask Congress for a declaration of war in April of that same year.

The United States had many reasons for entering World War I. The most important reason was Germany's unrestricted submarine warfare program was a danger to American lives and trade. Germany had already sunk several American merchant ships bound for Britain, and German attacks had killed hundreds of Americans before the declaration of war in 1917. The most famous German attack which almost pulled America into the war happened in 1915 with the sinking of the British ship Lusitania,which killed 128 Americans. After this sinking Germany pledged not to attack any more vessels unless they were explicitly purposed for war. Germany returned to unrestricted submarine warfare in 1917 in an attempt to force an early end to the war.  

Another outrage by Germany was the Zimmerman telegram, which was an attempt to get Mexico to attack the United States in order to distract the Americans long enough for Germany to win the war. In return, Mexico was supposed to get lands lost during the Mexican War. While the threat was not viable since the Mexican government was unstable, many Americans were outraged that Germany would even suggest this.

The United States had made many loans to the Allied powers before 1917. If the Allied powers lost the war, there was a strong chance that this money would never be paid back. British warships cut the German trans-Atlantic cable early in the war in order to make sure that only Allied-approved news made it to the United States. The Germans were portrayed as savages who committed atrocities in Belgium. When the Americans entered the war, soldiers enlisted to fight against the "Hun;" however, when they arrived in France, they reported few actual German atrocities. The war was barbaric on both sides.

Actually, there are more than two events.  British and French releasing anti-German propaganda in the United States played a role.  Also, the United States loaned the Allies millions of dollars to fight the war and if the Allies lost, then the money would not be repaid.  

The main reasons that America entered the war were that Germany resumed unrestricted submarine warfare against Britain and the leak of the Zimmerman note.  Germany waged unrestricted submarine warfare as early as 1915, then 128 Americans died on the British liner Lusitania when it was sunk in May of that year.  While it was carrying munitions for the Allied war effort in its hold, Wilson called this an attack against innocent civilians, and Germany issued an apology, paid an indemnity, and promised not to do it again.  By early 1917, German war planners realized that their window of opportunity for winning the war was closing, so they resumed unrestricted submarine warfare anyway with the logic that the war would be over before America could be a factor.  America declared war in April of that year after a few American merchant ships sank.  

As to the Zimmerman note, Arthur Zimmerman of the German Foreign Service in Mexico wanted to distract America from the European war, so he devised a scheme where Germany would help Mexico in a war against America in return for Mexico gaining back the land it lost during the Mexican war in 1848.  This was still in the planning stages when British intelligence intercepted the telegram and sent it to the American government, where it was quickly released to the press.  This stirred up more anti-German sentiment, especially in the West.  

America joined the war for four main reasons:

1.  German unrestricted submarine warfare was sinking American neutral vessels that were trading the Britain and France.  America almost joined the war over the loss of 128 American lives when a German U-boat sank the Lusitania in 1915.  Germany was forced to back away from its policy of sinking all vessels in British waters, but by 1917 Germany resumed this practice in order to try to win the war.  

2. The Zimmerman note, which was a secret deal between the German Empire and Mexico that would mean Mexico declaring war on America long enough to distract the United States from the larger war in Europe.  Germany pledged support, and after the war Mexico would receive the lands lost during the Mexican War fought against America from 1846-1848.  While the German government in Berlin was probably never aware of Ambassador Zimmerman's plan to make Mexico a nominal Central Power ally, British intelligence delivered the note to Washington, and furthered the idea that Germany could not be trusted.  

3. American war loans to the Allies would be in danger of not being paid back if the Allies lost the war.  Millions of dollars of aid in the form of loans and weapons sales were going to the Allied Powers well before 1917.  Germany appeared close to victory in late 1916 and early 1917, with Russia reeling in the East and the French military having mutinies on the Western Front.  Something had to be done to ensure that America's financiers received their money back with interest.  

4. Allied propaganda ensured that only one side of the story of the war was told.  Early in the war, the British cut the German telegraph cable to America.  From that point, the British ensured that only Allied war news was spread.  When Germany invaded Belgium, Americans heard of Germans attacking and enslaving civilians, when in reality this practice was not as widespread as reported.