Yes, I believe during WWII, the military allowed for service members to drink as a reward. Not sure if any such generosity was extended during Vietnam, or even during Korea.
Great story from a guy who was at my unit when I first arrived. While a young airman in the 80s, he attended tech school in a southern state with age 18 as the drinking age, and could therefore drink while off duty. He then arrived at a different southern state where the age was 21. His new commander, a good-ole-boy, got promoted from Lt Col to Col. While at the ceremony, this newly minted colonel swings by him, stops, and says, "son, I do not see you with a drink in your hand at my promotion ceremony." He responds, "sorry, sir, I can't. I'm not 21." This good-ole-boy responds indignantly, "son, this is my promotion ceremony, and at my promotion ceremony, you certainly will have a drink." He then places a drink in his hand, and wanders off.