In strategy games, to perform a rush, the attacking player focuses on quickly building a large number of weak units (or some very rare and potent early game units) early on in the game with the hopes of swarming the opponents before they can defend themselves; this is to catch them off-guard and often to cripple the opponent's economy structures. In the majority of cases, these units are fast and cheap to enable larger numbers and opportunistic attack strategies, but they may sometimes be chosen to exploit a particular weakness of the enemy. The player who rushes may sacrifice options such as long-term resource gathering, defense, or immediate research up the tech tree to opt instead for a quick strike, usually putting the rushing player at a severe disadvantage should the rush be unsuccessful.