How did World War I turn into a Global War?
World War I became a global conflict for several reasons. One reason was that many countries around the war were involved in some way with the war. The alliance system helped spread the war quickly. The two main alliances during the war were the Allied Powers and the Central Powers. While the emphasis is given to the main powers in each alliance, there were smaller countries allied with each side. For example, the Allied Powers included France, Great Britain, the Soviet Union, Italy, and the United States. Because Serbia was a long-standing ally of the Soviet Union, the Soviet Union felt it had to help Serbia when Austria-Hungary declared war on Serbia. This started a domino effect where allies of each nation joined the war to support their alliance members. Thus, a dispute that should have involved only two countries, quickly escalated because of the alliances that existed.
Many of the main countries in each alliance had empires that extended beyond their borders. This also helped expand the outreach of the war around the world. The places these countries controlled helped them fight in the war.
The stage was set prior to World War I for a small dispute between two countries to become one that involved many nations.
The assassination of Archduke Franz Ferdinand of Austria in Sarajevo, Bosnia by a Yugoslav national is an event in history that points to the start of the 1st World War.
The reason the war became a global war is because as stated by Pohnpei397 the countries involved had developed alliances. These alliances were used to maintain the global balance of power. The nations were linked by politics, trade among other factors making it difficult for a nation to ignore the plight of its ally.
Austria provoked a war with serbia after the assassination of the Archduke through a list of demands that were impossible to fulfill. This succeeded and ended up in Austria declaring war on Serbia. This in turn forced Serbia's allies to marshal up military support against Austria while the same was done by Austria's allies. This led to the involvement of countries such as France, Russia, Germany, Britain and their allies to get actively involved in the war.
Opportunists were not left behind and this is because this war created prospects for access to resources, forging of stronger alliances and to establish authority with regards to global operations and relations.
There are three ways in which this happened.
First, it happened because of the fact that there were far-reaching alliances on both sides. That meant that a dispute between Austria-Hungary and Serbia could draw in Russia and Germany and France and Britain.
Second, it happened because of the fact that many of these combatants had empires. The fact that the French had an African empire, for example, meant that many soldiers were brought from Africa to participate in the war. This is also why Australians and New Zealanders ended up fighting in Turkey.
Finally, it happened because various countries hoped to benefit from fighting. This is, for example, why a country as far removed from Europe as Japan got into the war on the side of the Allies.