Good supporters' group songs in MLS have a way of going viral. Everybody steals (borrows, improves upon, whatever) from everybody else.
And the song of recent vintage that's done the rounds the most is "I Believe That We Will Win." Since coming to MLS via the Red Bull Arena SouthWard in summer 2010 (and if you have evidence someone else in the league did it earlier, do go ahead and post it in the comments below), it's become a de rigeur chant.
Let's face it, though: KC does it best. The Cauldron absolutely owns this one.
Anyway, here's what our guy Shawn Francis over at TheOffsideRules called the song's "own, heart-fluttering short film," courtesy of the American Outlaws. It looks like it was shot primarily (completely?) at Livestrong Sporting Park and the surrounding drinking establishments before/during/after the recent US national team game vs. Guatemala, and it is pretty damn awesome: