The U.S. entered World War I because Germany (needlessly) DIRECTLY threatened U.S. interests.
The sinking of the Lusitania, with the loss of 128 American lives caused a lot of ill-will in the United States. And the resulting "unrestricted submarine" warfare was a threat to American notions of free trade going back at least to the War of 1812.
But the "last straw" was the Zimmermann Telegram, sent to "Mexico" proposing a "stab in the back."
http://history.stackexchange.com/questions/1031/was-the-zimmerman-telegram-a-ruse-on-the-us-or-a-way-of-germany-to-incite-mexicoWas the Zimmerman Telegram a ruse on the US or a way of Germany to incite Mexico to declare war on the US?
Mexico was in the throes of a civil war, and really had no central government. The Germans believed otherwise because one of the four major factions (under Pancho Villa) "invaded" the U.S. while fleeing from the others.
The U.S. would be threatened by a world in which Germany occupied Belgium and northern France (leaving a "rump" state in the south, as in World War II), probably northern Italy, and dominated the Balkans, Scandinavia, the Middle East, and the Baltic and East European countries near the Russian border, probably after uniting with Austria-Hungary (who had lost the heir to the throne when Archduke Franz Ferdinand was assassinated).
To have such a country allied with Mexico (or Brazil or Argentina) was too much, and a clear violation of the Monroe Doctrine.