United States of America

So why would a country an entire continent away join a European War? First off, America only joined in the last year of the war. Second, the main reason for joining was actually Mexico.

Well actually, it all started with the German policy of 'unrestricted submarine warfare' where the Germans shot all ships that were not their own. They ended up shooting an American Ocean Liner.

Whoops.

There came a demand for America to join the war in the interests of justice, liberty, freedom, etc. Still, President Woodrow Wilson kept a neutral stance.

Then came the news that Germany was helping Mexico regain some borders from America. That was the last straw, and America decided to enter the war.

Critics of America would argue that this was a vital step in becoming recognized as a World Power. The United States were enjoying their own 'splendid isolation' prior to the troubles of Europe. They did not want to unnecessarily waste American lives in a European war.