To countervail is to oppose something successfully.
To countervail is to counteract, counterbalance, or neutralize. This verb is best known to us in the form of its participle countervailing, which gets far more time in the limelight as an adjective than countervail gets as a verb. A favorite combo these days is countervailing duty, a duty imposed on imports to match (or retaliate for) what a foreign government is imposing. You could say a running back was countervailed if a defensive player stops him in his tracks.