And i don't remember Nixon withdrawing any troops.
Well, you have a bad memory.
The U.S. and other allied forces began drastically reducing their troop support in South Vietnam during the final years of "Vietnamization". Many U.S. troops were removed from the region, and on 5 March 1971, the U.S. returned the 5th Special Forces Group, which was the first American unit deployed to South Vietnam, to its former base in Fort Bragg, North Carolina.