Japan enters the War
Japan had a preexisting agreement with Britain, so on August 23, 1914, Japan declared war on Germany and seized much of Germany's Micronesian territories. Japan believed that the Allies would win, so they joined to gain some land from the German region. They invaded German territories within the Pacific, hoping to claim most of the land after the war, but only received smaller islands after the war ended. They also wanted to gain more recognition as a large power in postwar geopolitics, in which they were successful.
Causes for U.S. to enter the War: Warfare
One of the main reasons for the U.S. joining the war was the sinking of the Lusitania, a British transport ship that was bringing supplies from New York to Britain. There had been unauthorized German submarines along the American coastline, leading to anger from U.S. politicians. The German U-boat ring sought to halt any supplies from reaching their enemies, including Britain. The Lusitania had about 1195 people die, with about 128 of them being Americans. This was the final straw, leading the U.S. to join the war. The picture to the left shows the Lusitania sinking, with some survivors on safety boats watching in horror as people drowned. Also in the picture is a German submarine (right of the life boat), looking at their handiwork as the ship sinks.
Economic
The U.S. had huge investments with the French and British, about 2 billion dollars worth of payment, and if they could not pay America back, the economy would most likely collapse. France and England were financing their war through U.S. loans, and were also buying large quantities of weapons and other armaments. The United States wanted to ensure that they were payed back in full. The Germans also bought goods from the U.S. at first, but in much smaller quantities.
Political
Propaganda coming from both sides influenced the U.S. greatly as well. Woodrow Wilson did not want to enter the war, but after Theodore Roosevelt ran for another election term, he felt threatened and stated that the U.S. may be forced into warfare. If the United States entered the war, it would be able to show its strength and establish itself as a major world power. As well as WW1, a war between Mexico and the U.S. was taking place, but once the Great War began, Mexico forced American troops out of the country. After this event, a German telegram was intercepted stating that it would join forces with Mexico, leading to concern and outrage within the U.S.