In the case of WW II the undamaged US was able to deploy its industrial capacity to "restart" the other industrial countries, first in Europe with the Marshall

In the case of WW II the undamaged US was able to deploy its industrial capacity to "restart" the other industrial countries, first in Europe with the Marshall Plan and later subsidizing Japan as the "unsinkable aircraft carrier" to oppose the USSR during the Korean War. Eastern Europe and the USSR took longer to restart using their surviving industrial capacity. In the event of a global conflict it is likely to go beyond the point where it is impossible to restart the current industrial system quickly enough to support existing levels of population. The population is now much more reliant on sophisticated technical systems for support and those technical systems are much more complex and fragile than in the 1940s. Consider how few major fabs costing 10s of billions are producing the lions share of semiconductors. Even fewer plants produce the capital equipment and critical reagents needed to reconstruct them and resume processing. The same is true for other supply chains.