The Germans wanted to keep the United States out of the War where the British wanted the Americans in the War. Arthur Zimmerman sent a telegram to the Mexican government. That telegram proposed that if Mexico attacked the United States, the Germans would help retain the territories of Texas, Arizona, and New Mexico.
This action is a violation of the Monroe Doctrine.
In 1917, the Germans issued a declaration that they will resume unrestricted submarine warfare on any vessel approaching a enemy port. This essentially breaks the Sussex pledge that was agreed upon in 1916.
The Zimmerman Note, along with the resumption of unrestricted submarine warfare will cause President Woodrow Wilson to ask Congress for a declaration of War on Germany and the Ottoman Empire in April of 1917. The United States was now officially in the War.
The Americans arrived in Europe in late 1917 and joined the "allied powers" consisting of:
France, Belgium, Great Britain, and Russia.
The enemy, or the Axis nations were:
Germany, Austria-Hungary, and the Ottoman Empire among others.
Changes in Alliances
The Europeans had been fighting World War I for more than 2 1/2 years and had sustained massive casualties. The Germans and Russians had lost about 1.7 million soldiers, Austria-Hungary lost about 1.016 million, France lost about 1.15 million, and England lost about 880,000. No one had ever seen numbers like this in war.
When the Americans arrived in 1917, countries started to reassess their commitment to war. Russia was forced to drop out of the war because of the Bolshevik Revolution of 1917 (the October Revolution) which is the beginning of the Communist Revolution. Italy, who once sided with Germany, now switched sides and joined the Americans, British, and French. In many ways, the Germans stood alone in the trenches of the Western Front fighting the Americans, British, French, and now the Italians.
Americans in the War
In 1915, the size of the American army was only 500,000. Wilson needed to increase that number. He enforced the "selective service act" which essentially started the draft. The size of the military increased to 4.8 million of which, about 3 million served in World War I.
To limit resistance to the war, Wilson passed the sedition acts which suspended freedom of speech, freedom of assembly, and freedom of the press.