There were several reasons why the United States gave up the policy of neutrality to enter World War I. Two of the primary reasons had to do with threats due to the offensive tactics from the Germans.
Under President Woodrow Wilson, the United States remained committed to neutrality during the early years of World War I. The Germans began using U-Boats in the Atlantic Ocean. The Atlantic was frequently used to transport passengers and cargo on British and American ships. The Germans bombed several ships and Americans were killed. Despite this, the United States remained neutral and sought to negotiate with the Germans. In 1915, the Lusitania was bombed and sank. Over 1,000 passengers perished, including over 100 Americans. The US government continued their policy of remaining neutral and negotiating with the Germans. This pattern of negotiations following bombed ships continued despite public outrage in the United States. More and more ships were bombed in 1916 and early 1917.
The final turning point came when the Zimmerman telegram was intercepted. The information in the telegram revealed that the Germans were seeking to form an alliance with Mexico. If Mexico accepted the invitation to form an alliance, they would be able to reclaim the US states of Arizona, Texas, and New Mexico with help from the Germans. This information caused Wilson to change his mind about neutrality, and the United States entered World War I in April of 1917.
There were several reasons that the U.S. abandoned "splendid isolation" and entered the first World War.
One reason was moralistic. The U.S. saw itself as a beacon of freedom and democracy, two characterisitcs they felt Germany and its allies were at odds with. The attacks on Belgium, France and other nations stood in stark contrast to American ideals, so Wilson demanded that American enter the war and, "make the world safe for democracy"
Another reason which was more pragmatic was war loans. The U.S. had loaned Britain, France and their allies lots of money which, if they lost, might not be recouped. For this reason, the banking industry pressured the Wilson administration to enter the war in order to protect U.S. loans.
Still another compelling reason was the recent attacks by U-Boats on U.S. shipping. When Germany began using submarines to sink Atlantic shipping, the Americans were outraged since their ships were being targeted as well. The sinking of the Lusitania in 1917 was the final straw and probably the most compelling reason that the U.S. entered the war.
Finally, there was the issue of the Zimmerman Note. This was a diplomatic offer between Mexico and Germany where the Germans offered to help Mexico in a war with the U.S. which they hoped would detract from intervention by the U.S.