BALTIMORE — Baltimore was a city on edge Tuesday as hundreds of National Guardsmen patrolled the streets against unrest for the first time since 1968, hoping to prevent another outbreak of rioting. Maryland's governor said 2,000 Guardsmen and 1,000 law officers would be in place overnight to try to head off a repeat of the violence that erupted Monday in some of the city's poorest neighborhoods and sent a shudder through all of Baltimore. "This combined force will not tolerate violence or looting," Gov.