Strike a weapon [Standard][AoO: Yes]
Description: A combatant can use a melee attack with a slashing or bludgeoning weapon to strike a weapon or shield that a combatant's opponent is holding. The attacking weapon must be no more than one size category smaller than the weapon attacked. (Treat a buckler as Small, a small shield as Medium-size, a large shield as Large, and a tower shield as Huge.) Doing so provokes an attack of opportunity from the opponent because the combatant is diverting it's attention from the opponent to the opponent's armaments.
Then the attacker and the defender make opposed attack rolls. If the attacker wins, the attacker has made a successful attack against the weapon or shield.