First a little scene setting: The Seattle Sounders were already leading the Vancouver Whitecaps 2-0 when Kendall Waston came in with a hard tackle on Clint Dempsey. Watson won the ball, but also hit Dempsey's legs pretty hard. Whether or not you believe it was a fair challenge is likely rooted in how physical of a game you believe soccer should be.
In any case, Dempsey went down hard but was able to return. A few minutes later, he and Waston had this exchange:
This angle doesn't really show definitively whether or not Dempsey made much contact with Waston, but it prompted the two firey players to have the above exchange. Shockingly, it looks like Pa Modou Kah is the one who plays peacemaker and that's probably a good thing, as neither looked very likely to back down.
Of course, it was Dempsey who got the last laugh as he assisted Obafemi Martins on the Sounders' third goal shortly after this exchange.