[orthoxerox]

The USSR spent its demographic and industrial transition on this. When you have a booming population and a low industrial base, you can squeeze the people a lot harder: they still see that their lives are improving and the lives of their children will improve even more. This means you can continuously consume their savings via inflation, stoking the national economy.

South Korea famously did this very well, turning from a war-torn former dominion of Japan into an industrial and cultural powerhouse.

The Soviets went all-in on building a military-industrial complex without growing their civilian economy, basically eating all the gains their growing population provided. Very charitably, you could say they went all-in, expecting to win the conflict with the West and get their economies "for free", but badly miscalculated their chances.