Major powers have fought several wars since World War Two but those have typically been lower-intensity asymmetric wars fought against much weaker opponents.
Just because another major war is unthinkable, does not mean it is impossible. Multiple ongoing conflicts around the world could escalate into something more serious.
The closest any human society has come to a full war economy was the Soviet Union from 1941-45. The Eastern Front of that conflict was perhaps the most unrestrained and brutal theater of any war.
Since the advent of the hydrogen bomb, a thousand times more powerful than the atomic bomb dropped in 1945, the world lived what Albert Wohlstetter called a constant fear of total destruction.
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