Answer:
When World War I broke out across Europe in 1914, President Woodrow Wilson proclaimed the United States would remain neutral, and many Americans supported this policy of nonintervention. However, public opinion about neutrality started to change after the sinking of the British ocean liner Lusitania by a German U-boat in 1915; almost 2,000 people perished, including 128 Americans. Along with news of the Zimmerman telegram threatening an alliance between Germany and Mexico, Wilson asked Congress for a declaration of war against Germany. The U.S. officially entered the conflict on April 6, 1917.
World War I Begins
On June 28, 1914, Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife, Sophie, were assassinated by a Bosnian Serb nationalist in Sarajevo, the capital of the Austro-Hungarian province of Bosnia and Herzegovina.
One month later, on July 28, Austria-Hungary declared war on Serbia. Within a week, Russia, France, Belgium, Great Britain and Serbia had sided against Austria-Hungary and Germany, and the Great War, as it came to be known, was underway.
Germany and Austria-Hungary later teamed with the Ottoman Empire and Bulgaria and were referred to collectively as the Central Powers. Russia, France and Great Britain, the major Allied Powers, eventually were joined Italy, Japan and Portugal, among other nations.
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