To act against with equal force, power, or effect; to thwart or overcome by such action; to furnish an equivalent to or for; to counterbalance; to compensate
A countervailing force, power, or opinion is one which is of equal strength to another one but is its opposite or opposes it. Their strategy is expansionist and imperialist, and it is greatest in effect, of course, when there is no countervailing power. with an equally strong but opposite effect (contrevaloir, from contre- ( COUNTER-) + valoir )