Washington had been pouring troops into Vietnam since the early 1960s to back the South Vietnamese against Viet Cong guerrillas supported by the communist North.
But a major guerrilla offensive in early 1968 forced it to reassess.
Starting from the Vietnamese New Year holiday Tết in late January, thousands of communist forces attacked southern towns, including the cities of Huế and Saigon.
The surprise coordinated assault was ultimately beaten back, but it turned public opinion against US involvement in the conflict.
By late March, US president Lyndon B. Johnson announced a partial halt in US bombardments of the North.
It was the start of a long process of US disengagement in Vietnam, which culminated with the fall of the Southern capital Saigon in 1975 and the reunification of Vietnam in 1976 under the North.
Talks opened in Paris in May, as the French capital is being rocked by student protests.
Anti-war demonstrations that started on university campuses in the United States and Europe in the mid-1960s, with their chants of “US, go home!”, took on a new dimension in 1968.
Youths took to the streets around the world to vent anger at the war and the capitalist status quo, but also to demand sexual freedom, feminism, and protection of the environment.
In Germany, an assassination attempt in April against radical leftist student leader Rudi Dutschke unleashed a riot in Berlin. The unrest spread to dozens of German cities.
In France, students demonstrated in Paris on May 10, battling police through the night. Two days later, workers joined in, and a strike paralyzed the country for weeks.
President Charles de Gaulle dissolved the National Assembly on May 30, but his party came back even stronger than before in June legislative elections.
The social movement was echoed in Italy, Turkey, and Japan.
In Mexico, police cracked down on protesting students just ahead of the Olympic Games. Many were killed; officials put the toll at 33, while foreign witnesses said 200 to 300 people died.
There was more defiance at the Games: medal-winners Tommie Smith and John Carlos raised their fists on the podium in a Black Power salute that put the spotlight on discrimination against African-Americans.
It was a dark year for the fight against the racial segregation plaguing the United States. Martin Luther King, a black pastor and Nobel Peace Prize winner, was assassinated on April 4 by a white escaped convict.
His murder unleashed demonstrations across the country. Soon afterwards, President Johnson signed one of the last laws on civil rights demanded by King, an act that ended discrimination in housing.
On June 5, another political assassination rocked the United States: presidential hopeful Bobby Kennedy is shot by a Palestinian immigrant. The younger brother of president John F. Kennedy – himself assassinated in 1963 – died the following day.
The winds of revolt reached communist Czechoslovakia, where Alexander Dubček became head of the ruling party in January and tried to introduce reforms for “socialism with a human face”.
But the Prague Spring was unacceptable to Moscow, which still dominated communist Eastern Europe. In August it sent in tanks and soldiers, including from communist allies, that crushed hopes for change.
In 1968 the world awakened to the humanitarian disaster in Biafra, which was battling Nigeria to maintain the independence it declared the previous year.
Images of starving Biafrans emerged and mobilized a new kind of international humanitarian effort, leading soon afterwards to the formation of Doctors Without Borders.
The 30-month conflict and a Nigerian blockade eventually claimed a million lives, many from starvation.