WASHINGTON (AP) - The rapid spread of swine flu from Mexico surprised Pentagon officials, who had been focused on a possible Asian-borne pandemic in a response plan that would give the military a last-resort role in helping to impose quarantines and border restrictions.

Drafted and overhauled several times in recent years, the military's closely guarded plan for an influenza pandemic assumed that officials would have more time before the flu hit U.S. shores. The Associated Press obtained briefing documents about the military's pandemic contingency plan.

The H1N1 flu outbreak set U.S. military commanders scrambling to monitor and protect troops based near the 2,000-mile southern border and on ships nearby.

The virus spread quickly across the border into Southern California, infecting at least 27 sailors on a ship docked at San Diego, four Marines at Camp Pendleton and at least one Marine at Twentynine Palms. Several dozen Marines have been quarantined, and nearly two dozen other sailors had flu symptoms but have so far not been confirmed as having the H1N1 virus.

"We anticipated scientifically that we would have time to do different things," said Amy Kircher, an epidemiologist with U.S. Northern Command's surgeon general's office. Northern Command oversees the country's homeland defense, including coordination with Canada and Mexico.