I kinda feel like this question is purely opinion based. But in my opinion it was because of the Germans blockading Britian's ports. That led to the sinking of the Lusitania, some of them being American lives. German continued to sink more ships trying to pass the blockade. For the least impact part, it would have to be the fact that Germany made a deal with Mexico (Germany offered U.S. land in exchange in the war effort). This is purely opinion based, because the question itself is opnion based.
