Why did the US enter ww1?

Ok, let's first deal with some of the rubbish from earlier answers.
1. The RMS Lusitania was a British ship flying a British flag. The Germans had said much earlier that any ships flying a British flag were fair game. There were only 128 Americans on-board out of about 1600. The ship was sunk in 1915. USA entered the war in 1917.
2. The Lusitania sinking was not the main reason the USA joined the war but was simply ONE of the reasons. The USA demanded the Germans stop sinking passenger ships. The Germans stopped (for a while)
3. The USA pursued a policy of isolationism (surprise, surprise) and wanted to keep out of the war.
4. Pearl Harbour was in WW2 not WW1.
5. There were no Nazis in WW1.

Why did the U.S. enter WWI?
1. It was starting to look like Germany was going to be defeated. The USA wanted to be part of the post war carve up and wanted to be there for their share of the pie.
2. The USA was supplying war materials to the allies. They could not do this and be "neutral" as well. In fact the USA was breaking international law by supplying the allies.
3. German agents committed two acts of sabotage on war materials in the USA destined for the trenches of Europe.
4. American public opinion was slowly turning against Germany and the public believed the USA should help France and Britain.
5. The American military was very keen to have a go at the "Hun".
6. The Zimmermann Telegram and the sinking of seven American merchant ships.
7. Britain softly manipulated the USA into joining the war (refer "6"). History can't really blame them.