The long-term trend that had the most to do with the US entering WWI was its trade relations with Great Britain. The US and Britain traded much more than the US and Germany did. This predisposed the US to side with Britain. Another trend that had something to do with this was the Progressive trend towards trying to improve the world. This made Americans responsive to Wilson’s idea of making the “world safe for democracy.”
There were also immediate catalysts. The most important of these was the unrestricted submarine warfare that the Germans were waging. Another was the Zimmermann Note, in which Germany tried to induce Mexico to enter the war by promising that Mexico would get back territory taken by the US in the Mexican War.