Motivation from the French swimmers’ pre-race boasts that they would "smash the Americans" didn’t hurt, but I believe the prime reasons the US team won the relay was their technique and pacing.
Many will focus on the incredible final 50 of Jason Lezak (he made up .6 on the 100m WR holder on that lap) as the reason for the win, and I would certainly nominate it as the best relay leg of all time. However, it’s important to bear in mind that the way Phelps, Weber-Gale and Jones swam their legs — as well as how Lezak swam the first 75 meters of his — made it possible for Lezak to be in position to pass Bernard at the end.
The US swimmers won this time for the same reason they lost the same relay to the Australians in 2000. That time, every US swimmer overswam the first 50 and were overhauled and left behind by their Ozzie counterparts on the 2nd 50. The psychological impact of "eating wake" on a relay leg is devastating. Eight years ago I wrote on the TI Forum that it was hard to believe four swimmers accomplished enough to swim the marquee relay for the US at the Olympics could all make such a rookie mistake. From an early age, swimmers are coached (at least those who receive good coaching) to draw energy from, but not be overwhelmed by, the emotion of swimming a critical relay leg. Emotion can carry you to incredible heights; it can’t overcome the physical/energy limits of poor pacing.

This time it was the French team, which swam like overeager rookies and the US swimmers who paced their 100m legs impeccably. it was a complete turnabout, with every French swimmer pulling away strongly on the first 50 and being hauled in – and in Barnard’s case overtaken. (Go to nbcolympics.com and watch again. Phelps set the pattern when he nearly ran down Eamon O’Sullivan of Australia on the first leg, even as Sullivan was breaking the world 100m record.)

And an essential part of the reason for their strong finishes was the far superior technique they displayed. The French team was characterized by super-high stroke rates and massive emphasis on overpowering the water. The US swimmers maintained far longer and smoother strokes. Phelps and Jones in particular are noted for their ability to maintain elegant form at top speed. This photo [above] of Phelps from his 100m leg illustrates the low head position, compact recovery and even a relaxed forearm and hand — as evidenced by the way his right wrist yields to water pressure — taught in Lesson 5 of the Easy Freestyle DVD.