The United States became involved in World War I for two main reasons, the Zimmerman telegram and unrestricted submarine warfare by Germany. The Zimmerman telegram was a coded message asking Mexico to join Germany’s side in World War I. Germany promised that if Mexico joined them they would get their lost territories back from the United States. Mexico had lost Arizona, New Mexico, and Texas. Germany wanted a war between Mexico and the United States because it would have tied down the American military forces so that they could not have sent men to World War I. The Americans had been providing arms to the allies and Germany wanted this practice to stop. The British managed to intercept the coded message from Germany's Foreign Minister Arthur Zimmerman to Germany's ambassador in Mexico in 1917.

The British shared the contents of the Zimmerman telegram with the United States and the American public was outraged. Americans felt that the war was being brought to their own land and it did not seem so far away anymore. The Zimmerman telegram helped change public opinion in the United States to join World War I with the allies. Within the secret coded message Zimmerman revealed Germany's plan to restart unrestricted submarine warfare as well. A German submarine had attacked the liner Lusitania in 1915 where 128 Americans died. After this attack Germany promised not to attack passenger ships anymore.

 

The Germans were desperate by 1917 to renew submarine warfare in an attempt to cut off trade to Britain. Germany needed to stop the supply route to Britain with their submarine attacks. They were aware that this would force America to enter the war. That is why Germany offered land to Mexico in return for Mexican support of war against the United States. The submarine's primary targets were merchant convoys that brought supplies for the war effort. From 1914 to 1918 Germany U-boats sank about 5000 ships. The German Empire relied heavily on imports for food and raw materials. The British declared the whole North Sea a war zone and could fire upon any boat. The British relied on imports to feed its population as well. The British ships had superior numbers and could operate throughout the world's waters.

The United States entered World War I in April of 1917 until the end in November of 1918. The United States sent 4 million military personnel. The US joined the Allied powers and suffered 110,000 deaths in World War I.