HO CHI MINH CITY, Vietnam — As the Marines scrambled to the roof of the U.S. Embassy, they locked a chain-link gate on every other floor to slow the throng of panicked Vietnamese civilians sure to come behind them. They knew if the crowd pushed through to the top, they could easily be overrun by hundreds of people desperate to get a seat on one of the last helicopters out of Saigon. The men barricaded the rooftop door using heavy fire extinguishers and wall lockers and waited nervously as Vietnamese gathered outside rammed a fire truck through an embassy entrance.