Synonyms for Counterattack:

n.

go for, fly at, mug, ambush, assail, strike at, come at, set on. act (noun)
countermove.
counteraction (noun)
counterstrike, countermove, an eye for an eye.
counterattack (noun)
countermove, counterstrike.
defense (noun)
countermeasure, protection, defense, moat, security, stronghold, harbor, argument, aversion, fortress, wall, rebuttal, armor, response, barrier.
opposition (noun)
defiance, protest, interference, friction, disagreement, obstruction, confrontation, contradiction, challenge, fight, hindrance, suppression, objection, antagonism, rebuff, adversary, resistance, repulsion, dispute, opposition, counteraction, conflict, repression, inhibition, obstinacy, impediment.
retaliation (noun)
counterstrike, countermove, vengeance, reprisal, retribution, revenge, counterblow, an eye for an eye, reciprocation, counteraction.

v.

competition (verb)
counterstrike.
defend (verb)
shelter, oppose, answer, argue, reply, police, uphold, retort, buffer, backfire, fend, avert, shield, save, protect, bar, counter, defend, resist, safeguard, support, return, barricade, guard, screen, cover, sustain, secure, respond, ward, rebut.
oppose (verb)
counteract, check, object, hinder, repress, oppress, impede, suppress, antagonize, confront, intercept, meddle, inhibit, repel, interfere, repulse, contradict, obstruct, cross, disagree, defy.

Other synonyms:

pounding, salvo, onslaught, tanning, swoop, revenge, counterblow. tit for tat, retaliation, requital, vengeance, reciprocation. aggression, retribution. assault, attack. charge. retribution
reprisal.
Other relevant words:
countermove, reprisal, requital, counterstrike, tanning, attack, a tooth for a tooth, retaliation, measure for measure, counterblow, retribution, aggression, reciprocation, assail, salvo, onslaught, revenge, vengeance, charge, like for like, assault, an eye for an eye.