Which statement explains this rise in the number of u. s. troops in vietnam during the 1960s? during the 1960s, several communist controlled nations joined the vietnam war on the side of the vietcong. despite u. s. efforts, south vietnam could not prevent a takeover by north vietnam on its own. president johnson worried that a small presence in vietnam would encourage soviet aggression in the region. military commanders warned that a civil war would break out in vietnam without a sizeable peacekeeping force in place..