Why did the United States enter the Great War?

Absent a credible threat of invasion, what was the reason that America declared war on Germany? Was it "protection of shipping," was in making the "world safe for democracy," was it stamping out "Prussian militarism?"

Don't forget the impact of the Zimmerman Telegram. A combination of factors was the cause, I think, including the ones you listed above.

The Zimmerman Telegram - I believe - only pledged Germany to support Mexico in taking back part of the previous Mexican territory if Germany declared war on the U.S.

I guess I am searching for a "good," i.e. valid reason why Wilson declared war.

>3 Blythewood: 'Only'? The Germans asked for bases for submarines in Mexico, access to Mexican oil, offered them weapons and sent military officers to train the soldiers. Basically Germany suggested, financed and supplied an invasion of the United States, plus they admitted they sent the telegram. That was sufficient. They also declared unrestricted submarine warfare on all shipping, neutral or not. The loss of American lives via the ships that were sunk was not negligible.