A counterattack is a military tactic used by defending forces when they had succeeded in halting an attack by an enemy force. During their assault, the attackers may have become vulnerable through exhaustion, complacency, or placing themselves in unfavorable tactical position. When the attack is seen to have exhausted the offensive momentum, the defenders aggressively charge out of their positions, often previously fortified, meeting and attacking their enemy either directly or by envelopment, to gain the initiative. This can break a siege or change the tide of an engagement. However, if the wrong moment is chosen, when the attack had not exhausted reserves or had been able to assume defensive positions, or a counterattack is poorly executed, the counter-attack can and will falter, since the defenders are no longer protected by their defensive positions.