The period between 1960 and 1968 also marked a cold period in the political relations between Sweden and the U.S., mainly due to the Swedish government's vocal opposition to the Vietnam War. In February 1968, the US recalled its Ambassador from Sweden after the Swedish Minister of Education and future prime minister Olof Palme, a Social Democrat, had participated in a protest in Stockholm against the war together with the North Vietnamese Ambassador to the Soviet Union Nguyen Tho Chan. The post of US Ambassador to Sweden remained vacant until February 1970. In December 1972, Olof Palme (then Prime Minister) made a speech on Swedish national radio where he compared the ongoing US bombings of Hanoi to some of the worst atrocities committed by the Nazis. The US government called the comparison a "gross insult" and once again decided to freeze its diplomatic relations with Sweden (this time the freeze lasted for over a year).