Dozens of gunmen mounted rare and apparently coordinated attacks targeting two army garrisons in northern Mexico, touching off firefights that killed 18 attackers. The attempts to blockade soldiers inside their bases part of seven near-simultaneous attacks across two northern states appeared to mark a serious escalation in Mexico's drug war, in which cartel gunmen attacked in unit-size forces armed with bulletproof vehicles, dozens of hand grenades and assault rifles. While drug gunmen frequently shoot at soldiers on patrol, they seldom target army bases, and even more rarely attack in the force displayed during the confrontations Tuesday in the border states of Tamaulipas and Nuevo Leon areas that have seen a surge of bloodshed in recent months.