Demonstrations took place across the country, including on college campuses.

AUDIO: Kent State shooting

On May fourth, nineteen seventy, National Guard troops shot and killed four students during protests at Kent State University in Ohio.

This is what Neil Young means when he sings of "four dead in Ohio" in a famous protest song that he wrote in reaction to the killings.

(MUSIC: "Ohio"/Crosby, Stills, Nash and Young)

President Nixon said again that the United States must not permit North Vietnam to take over South Vietnam. Lyndon Johnson had said the same thing when he was president.

Many Americans had accepted the war, but as it continued, and as more Americans died in Vietnam, public opinion changed.

In nineteen sixty-five, sixty-one percent of those questioned said they approved of the war. By nineteen seventy-one, sixty-one percent said they did not approve of it.

The official peace talks in Paris offered little hope of a negotiated settlement. Over a period of several years, each side made proposals, only to have the other side reject them.

President Nixon wanted to address the public's anger over the war. So he announced that Henry Kissinger had held twelve secret meetings with North Vietnamese officials. But those secret meetings made no more progress than the official talks.

In late March nineteen seventy-two, North Vietnam launched a major offensive. In May, Nixon ordered increased bombing against roads and railways in the north. By the end of August, the communist offensive had been stopped. Yet many lives had been lost. The pressure to withdraw American forces grew stronger.