Germany entered World War I because countries that were its allies entered the war first. Germany was eager to go to war, but it did not officially do so until it had a way to justify doing so.
Many historians feel that Germany was the main cause of WWI. They say that Germany wanted to expand its power and took aggressive actions that scared other countries in Europe. They argue that the German actions provoked other countries to form alliances against Germany. Even if Germany was the main cause of the war, it did not enter the war until other countries had already declared war on one another.
The first country to declare war in WWI was Austria-Hungary. That country issued an ultimatum to Serbia after the assassination of the Austro-Hungarian Archduke Franz Ferdinand. When Serbia rejected the ultimatum, Austria-Hungary declared war on July 28. Austria-Hungary was an ally of Germany. Russia was an ally of Serbia and it mobilized its army when Austria-Hungary declared war. It said it was mobilizing against Austria-Hungary only and not Germany, but Germany declared war on Russia anyway. It was ostensibly entering the war because Austria-Hungary was its ally and Austria-Hungary was at war with Serbia and being threatened by Russia.
Germany’s main allies in this war were Austria-Hungary and the Ottoman Empire. Bulgaria was also involved in the war on Germany’s side but was not a major player. Italy was on Germany’s side at first but switched sides because it wanted to take part of Austria-Hungary’s territory.
Germany was not the instigator of World War I, but it was a key player in the war. Germany signed a treaty with Austria-Hungary promising support if the nation was threatened. After the assassination of the Austrian Archduke Franz Ferdinand by Serb nationalist Gavrilo Princip, Austria-Hungary gave Serbia a list of unreasonable demands to meet or else it would be invaded. Serbia, a Slavic nation, appealed to Russia, as Czar Nicholas II claimed to defend all of the Slavs. Russia started to mobilize in an attempt to intimidate Austria-Hungary into backing down. Austria-Hungary secured German backing for its own mobilization. Germany feared Russian mobilization as well and it declared war on Russia. France, bound by treaty to defend Russia, then entered the war against Germany.
The Central Powers included Germany, Austria-Hungary, the Ottoman Empire, Italy, and Bulgaria. The Ottoman Empire was in this coalition because it was angry at Russia after the Balkan Wars and wanted to regain territory lost. Germany also hoped that the Ottoman Empire would threaten British interests. Italy, a charter member of the Central Powers, would switch sides because it coveted Austro-Hungarian land.
World War I was precipitated by the assassination of Archduke Franz Ferdinand, heir to the throne of Austria-Hungary, in Sarajevo on June 28, 1914. A Serbian nationalist named Gavrilo Princip shot the archduke, and this action caused Austria-Hungary to declare war on Serbia on July 28, 1914. Then, a series of countries were dragged into the war because of their alliances. Russia began to prepare for war, and, after delivering an ultimatum to Russia to stop its mobilization for war (Russia refused), Germany declared war on Russia on August 1. Germany became entangled in the war because it was the ally of the Austro-Hungarian Empire. The Central Powers went on to include not only Germany and Austria-Hungary, but also the Ottoman Empire and Bulgaria. On the opposing side, the Allied Powers included France, Great Britain, Russia, Serbia, and other countries, including, eventually, the United States.