Nearly 20 years after the Vietnam War's end, U.S. diplomats hoisted the American flag today in Hanoi, christening their newly-opened liaison office and marking a new political and commercial relationship with the Communist country. "I didn't even know it was an American flag," said one Vietnamese onlooker. "This is the first time I've ever seen one." The U.S. set up the liaison office as a transitional step toward full diplomatic relations, provided Vietnam does more to help solve the cases of the 2,211 U.S. servicemen still missing in the conflict. No U.S. flag has flown over a government building in Hanoi since Washington severed ties in 1955, a year after Ho Chi Minh seized power from French colonizers.