The primary events that led to the United States declaration of war against Germany were the Zimmerman Telegram and Germany's announced intention to resume unrestricted submarine warfare.
American sentiment had leaned toward the Allies and against the Central powers for some time. Americans felt a common affinity toward the British because of the common language. Also Woodrow Wilson made no attempt to hide his disdain for persons of German ancestry. He once referred to German Americans as "hyphenated Americans." Sentiment against the Germans was also intensified after the sinking of RMS Lusitania in 1915; however this was NOT the cause of U.S. entry into the war. After that event President Wilson famously commented "there is such a thing as being too proud to fight."
Following the sinking of the Lusitania Germany had issued its Arabic Pledge (sometimes known as the Sussex Pledge) in which the High Command promised it would no longer sink Allied ships without first giving appropriate warning. Several things changed this: The British often few the flags of neutral countries on their ships, and also rammed German U-boats while pretending to allow boarding. Then too the war was at a stalemate, and the Germans had to do something to move the war effort. This led to their notice on January 31, 1917 that Germany would resume unrestricted submarine warfare the next day.
On February 25, 1917, President Wilson received an intercepted German telegram to the German Embassy in Mexico City that offered to Mexico the "lost territory" of Arizona, New Mexico, etc. (land lost by Mexico to the U.S. in the Mexican American War of 1848) if Mexico would declare war on the U.S. The British had intercepted the telegram, and delivered it to the U.S. in an obvious attempt to secure U.S. entry into the war on the Allied side. On March 17, 1917, Germany sank five U.S. merchant vessels, and President Wilson delivered a war message to Congress on April 2. War was declared on April 6, 1917.
There are two main factors that led the United States to enter World War I after having stayed neutral for so long.
First, there was the fact that the Germans had resumed unlimited submarine warfare. This had been a point of contention between Germany and the US for most of the war. The Germans had stopped using the tactic for a time, but started up again in January of 1917. A number of American ships were sunk in March of that year, leading to the declaration of war in April. The US felt that this type of warfare was illegal and inhumane. The government also did not like the fact that it reduced American trade with Britain.
The other immediate cause was the Zimmermann Telegram, which tried to persuade Mexico to enter the war so as to keep the US tied down at home and unable to help the Allies in Europe. This angered many Americans. Americans were particularly angered by the fact that the Germans offered to give Mexico land that the US had taken from it in the Mexican-American War.
These two factors led to the end of US neutrality in this war.
The zimmerman telegram which tried to make mexico attack the usa, and also the fact that the axis were sinking u.s.a boats using submarine warfare. The events led to a change of opinion about the war and people started encouraging the usa to join the war.
The United States, led by President Woodrow Wilson, did not want to be involved in World War I. When Wilson ran for re-election in 1916, his campaign slogan was "He kept us out of the war" referring to the European conflict that was then referred to as the "Great War." However, by 1917, sentiment was changing with regard to the war.
Although the U.S. had yet to become officially involved in the war, most Americans supported the Allies and saw the Germans as aggressors. Americans felt a connection to Great Britain, in particular, because of cultural similarities. Germany angered Americans with their U-boats sinking American ships suspected of aiding the Allies. With the sinking of the Lusitania in 1915, a British cruise ship, 128 Americans were among those who lost their lives. The Germans temporarily halted such actions but in 1917 resumed unrestricted Naval warfare. At the same time, the British intercepted a message, called the Zimmerman Note, asking the government of Mexico to declare war on the U.S., if war broke out between the U. S. and Germany. The note also promised to help Mexico regain the territory of Texas, Arizona, and New Mexico if Mexico declared war on the United States. This note was the final push that Wilson needed to turn public sentiment towards war. The U.S. Congress declared war on Germany on April 6, 1917.