Johnson's biggest problem was: ?
congress
credibility
the vietnam war
the cambodian purges

Johnson's biggest problem was the Vietnam War.

Explanation:

On the night of August 4, 1964, President Lyndon B. Johnson, sitting in his White House office before the television cameras, testified that two days earlier, the destroyer USS Maddox had been attacked by North Vietnamese torpedo boats in the Gulf of Tonkin. He said, likewise, that a few hours earlier - from that day 4 - other North Vietnamese torpedo boats had attacked, in the very waters of the gulf, the destroyer USS C Turner Joy. Finally, he added that neither the Maddox nor the Turner Joy were engaged in any hostile activity towards North Vietnam, nor were they involved in intelligence work to support the attacks that the South Vietnamese torpedo ships were perpetrating against the coastal facilities of their northern neighbor.

Three days later, the United States Congress passed the Gulf of Tonkin Resolution, which authorized the Johnson government to use all the necessary force to "punish" North Vietnam for that aggression and to intervene, openly, in the war.

Lyndon Johnson accepted the so-called "domino theory" that posed that the fall into communist hands of South Vietnam would be the first piece of a wave of communist advances in Asia. When he became president in 1963 there were just over ten thousand American soldiers, three years later the troop number for Vietnam amounted to half a million men.

The Vietnam War was much discussed. In 1968, the United States had 548,000 soldiers in Vietnam, 30,000 of them had fallen in combat. Johnson's approval rates, which hovered around 70 percent in mid-1965, dropped dramatically to less than 40 percent in 1967.