The End of Neutrality

WWI, or the Great War, was an international war that had a devastating impact.  Although the fighting began in Europe, European nations involved much of the world in the Great War.  One of these reasons was imperialism - European nations relied on their conquered territories as a source of manpower.  For example, the British used the Indian army to fight against the Central Powers.

Indian soldiers marching off to war in WWI.  Although a number
of Indian soldiers died in combat, their contribution to the war
remains largely forgotten today.

America, however, was not one of these imperialized countries.  It was one of the "great civilized powers" of the time.  Roosevelt relished its ability to stay neutral in a time when the rest of the world was involved in the devastating war.  US leaders were content with keeping trade open to both sides, which benefited farmers and businesses who were still recovering from an economic drag.  Members of the peace movement also supported the neutrality policy.  However, these supporters could not stop America's involvement in WWI.

America's involvement in WWI was a combination of nationalist sentiment and the need for economic self-preservation.  Nearing the end of WWI, Germany and Britain were stuck in a deadlock and sought to starve their enemy.  Both Germany and Britain did their best to stop all ships from reaching their enemy, even if they were neutral.  However, America primarily supported the Allies for two reasons.  First, Britain had strong economic ties with the US, and American businesses made more trades with the Allies than with the Central Powers.  Secondly, Germany began using U-boats to tip the scales in their favor, declaring waters around Britain a war zone.  When they fired upon and sank the British Lusitania, causing a number of American deaths, which incited anti-German sentiments in America.  Germany initially agreed to America's demands to not fire on merchant ships.  However, Germany later announced unrestricted submarine warfare, in an all-out attempt to win the war, which caused America to cut ties with Germany.

A U-Boat.  Short for "undersea boat", U-boats served as an
important tactical device.  They were used in an attempt to
break the British blockade and block supplies from reaching
the Allies.

A U-boat sank the RMS Lusitania in May 1915.  There were
128 American deaths as a result of this attack.

Aside from the sinking of the Lusitania, the other major event was the Zimmerman Telegram.  Propaganda spread by the Allies in America was highly effective at sowing anti-German sentiment.  The Zimmerman Telegram was a telegram sent to a German Minister in Mexico that was intercepted by Britain.  It suggested that Germany ally with Mexico if the US entered the war, in return for helping Mexico regain territory lost to America.  This note stirred the discontent throughout the nation.

The Zimmerman Telegram was a German note sent in code.
The British got hold of this note and quickly released it to
the United States

Both nationalist sentiment and economic desire led to the US entering the Great War.  In April 1917, the United States formally declared war against Germany.