Though perhaps inevitable that the United States would be forced to join World War I eventually, the natural assumption that the US would automatically fight on the side of the Allies is incorrect. Throughout the late 19th and early 20th centuries, the United States had made an effort to remove itself from European politics and conflicts. Thus, when tensions in Europe began to escalate, the US could have sided with the Germans as well. However, there were reasons for the United States to side with the allied powers and reject the Germans attempts at an alliance.
In February of 1915, Germany created naval blockade of Great Britain. The Germans would sink any ships, even neutral ones, until Great Britain allowed trade between Germany and other nations once more. By this point, Germany was employing U-Boats, which created problems for neutral ships. International law required an attacking warship to war merchant and passenger boats of impending attack, however by doing so; a submarine gave up its unique element of surprise.
A WWI Recruitment poster, using a New York Herald ...
German soldier's "Auf Wiedersehen" (LOC)
Germans at Princeton -- Weitzer - Lt. Reichenau - ...This meant that German U-Boats hoping to sink their targets and avoid being sunk themselves could not afford to abide by the international law. In March of 1915, the Germans sunk a British ship, killing an American in the attack. The United States couldn't decide upon the proper course of action, however in May of 1915, the Germans sunk another ship carrying American passengers. The Lusitania was a British luxury liner, and it was carrying both passengers and ammunition. Approximately 1200 people were killed, 128 of whom were Americans. In response, Americans began calling for a declaration of war on Germany. However, naval relations between the United States and Germany settled down after the German ambassador promised that the German navy would give warning to ships about to be attacked. Though...