Why did America enter the First World War?

The United States had many reasons for entering World War I. The most important reason was Germany's unrestricted submarine warfare program was a danger to American lives and trade. Germany had already sunk several American merchant ships bound for Britain, and German attacks had killed hundreds of Americans before the declaration of war in 1917. The most famous German attack which almost pulled America into the war happened in 1915 with the sinking of the British ship Lusitania, which killed 128 Americans. After this sinking Germany pledged not to attack any more vessels unless they were explicitly purposed for war. Germany returned to unrestricted submarine warfare in 1917 in an attempt to force an early end to the war.  
Another outrage by Germany was the Zimmerman telegram, which was an attempt to get Mexico to attack the United States in order to distract the Americans long enough for Germany to win the war. In return, Mexico was supposed to get lands lost during the Mexican War. While the threat was not viable since the Mexican government was unstable, many Americans were outraged that Germany would even suggest this.
The United States had made many loans to the Allied powers before 1917. If the Allied powers lost the war, there was a strong chance that this money would never be paid back. British warships cut the German trans-Atlantic cable early in the war in order to make sure that only Allied-approved news made it to the United States. The Germans were portrayed as savages who committed atrocities in Belgium. When the Americans entered the war, soldiers enlisted to fight against the "Hun;" however, when they arrived in France, they reported few actual German atrocities. The war was barbaric on both sides.