Under the Wilson Administration, the United States government did its best to remain neutral during the First World War. Many Americans saw the war as a European affair that was of no concern to them. But as the war progressed, various factors and events put enough pressure on the government that it declared war on Germany in April 1917.
It should be mentioned that there were some Americans that lobbied for war very early on. Former president Teddy Roosevelt argued in support of the Allies from the very start. He and others felt that Germany had overstepped with its invasion of Belgium and attacks on international shipping. However, President Wilson maintained the country's pledge of neutrality.
That neutrality became harder to defend as the war progressed. A large part of this was the result of Germany's use of unrestricted submarine warfare against Great Britain. This greatly threatened American economic interests as a participant in international trade. Furthermore, it was costing American lives. Numerous American sailors and passengers died in submarine attacks. After several high-profile submarine attacks that resulted in numerous American casualties, such as the sinking of the Lusitania in 1915 and the Sussex in 1916, a lot of pressure was put on the government to do something to protect American lives and punish Germany. In May 1916, Wilson received a pledge from Germany that it would end such attacks. However, when Germany broke the conditions of the agreement, the clamor for war became even louder. Many newspapers published editorials supporting intervention. Even quite a few politicians publicly echoed these sentiments.
Perhaps the event that pushed the United States to the point where neutrality could no longer remain an option occurred with the so-called Zimmerman Telegram. This was a German correspondence to Mexico pledging to support them if they declared war on their northern neighbor. Mexico, busy with its own revolution at the time, rebuffed this offer. However, British intelligence intercepted the telegram and turned it over to the United States. Once the contents of the telegram were made public, Americans' calls for war became too loud to ignore.