In the case of Indochina (Viet Nam, Laos, and Cambodia), it was the US who proposed that Indochina be under UN trusteeship towards independence. The italicized words are keyword search. Use them.
Roosevelt died before his idea was even formally proposed, however, Truman had no cause to stop supporting it. When Mao won the post WW II Chinese civil war, he began to make eyes towards Viet Nam, prompting Ho to negotiate with France for the infamous but often suppressed knowledge of the Ho-Sainteny Agreement that placed Viet Nam under French protectorate status. With communist incursions into Viet Nam and a backstabbing ally, Truman had no choice to quash the UN trusteeship idea. To nominally 'support' France in SE Asia was a necessary evil in the anti-communist calculus. Even today, most Viets, including those overseas, do not know that it was Ho Chi Minh who literally invited France to return to Viet Nam. If Mao had kept China out of Viet Nam, there would have been no Vietnam War as we know it.