The mistake was allowing the French to keep Indochina after WWII.
The French warned us they couldn’t keep Indochina; their public wouldn’t stand for the war, and they had no money to pursue it. We assured them we’d fund it, sent a ton of weapons and supplies (including an aircraft carrier, re-named and staffed by a French crew), and then left them to their fate at Dien Bien Phu (despite assurances of support). French troops fighting in Indochina looked just like American troops fighting simultaneously in Korea; the uniforms, helmets, weapons, radios - all the same.
The worse betrayal of all was the Americans agreeing to an armistice in Korea in 1953 (despite French please not to do so); almost immediately, Red Chinese weapons that had gone to North Korea showed up in Indochina - especially the heavy artillery that demolished Dien Bien Phu.
Right through the very end of that war, the French government (unlike the US ten years later) was not allowed to conscript a single Frenchman to fight in Indochina; the war was fought with colonial African troops, native Indochinese, the Foreign Legion, and the small number of French volunteers from the regular army. Those colonial troops returned home after the war and led their own independence movements, costing the French much of what remained of the rest of their empire.