In 1914 war broke out across Europe. It began with the assassination of Francis Ferdinand, the archduke and heir to the Austria-Hungarian throne. His assassinators were Serbian revolutionaries. Soon Austria-Hungary was in conflict with Serbia and alliances were being drawn up left and right. Just like that, a Europe that had been industrializing and militarizing for years, was ready to go to war with its self. Each nations' reasons for becoming involved in the war vary, but it was strictly a conflict that involved the continent of Europe and it's possessions.
In 1915 a ship carrying American passengers had nearly finished crossing the Atlantic when it was torpedoed without warning. Out of all the passengers who died, one hundred and twenty eight of them were United States citizens. The ship was called the Lusitania and it was destined for Britain when a German U-boat sank it with all but one torpedo. Now the Lusitania was a passenger ship traveling in neutral waters when it was attacked. So the people of the U.S. were understandably upset over the German's aggressiveness, but one needs to look at this for what it is.
Before the ship even left port, before the tickets were even bought, warnings were posted explaining the potential risk people were putting themselves in. Even if the passengers happened to miss the warnings that were posted, they surely could not have missed the news. The war was all over the media, the passengers knew about the dangers and the deadly U-boats that patrolled the northern Atlantic. Why was it such a shock? Why was it so devastating to the American public when they heard the news?
Regardless of the sinking, the United States continued to remain neutral. The U.S. maintained the policy that they would not get involved in Europe's affairs. It was their war, their problem. Then came the Zimmerman Note. The Zimmerman Note was a telegram sent by the Germans. On its way to Mexico, Britain intercepted the telegram and sent a copy the United States government. The telegram addressed the Mexican government asking them to invade the Southwestern portion of the United States. In doing so, Germany promised them that they would help return all the territory taken from them by the U.S.
Still, even with Germany's attempt to stage an invasion on the United States, President Wilson and his congress maintained their neutrality. But then, later on in 1917, Russia withdrew from the war. This meant that Germany no longer needed to have nearly as many troops along the Eastern Front, so they could move them to where they really needed them, to the Western Front. Now Germany really had the upper hand in the war. It was now, that the United States of America joined the Allies.
In a nutshell, a neutral ship in neutral water is sunk and one hundred and twenty eight Americans die. The United States stays neutral. An attack is planned out against the United States. The United States still remains neutral. Why did the United States remain neutral? It wasn't their war; it wasn't their conflict, not their business, not their problem. Then all of a sudden, in 1917, the United States declares war on the Central Powers. For three years the United States stayed away, then they join in. The United States had no reason to enter the war. Their economy was booming, they had no desire to acquire land and they had a completely indifferent view of what was going on in the war.
In the aftermath, even, where did the United States benefit? No one paid any mind to President Wilson's Fourteen Points and all the U.S. got out of it was a bunch of casualties. It does not seem altogether that clear why the United States got involved and looking back, perhaps it would have been wiser to stay neutral throughout the war.