Ukraine

From a purely military perspective, active combat creates expertise at all levels in a military organization. Those that survive improve. This remains true until the losses outcompete the ability of the military to train replacements.

NATO forces have little experience with the type of war that Ukraine is facing. How do you fight all the inexpensive drones being mass produced? These drones have proven to be able to damage or destroy just about everything on the battlefield.

It's a whole different war that will lead to rapid developments in new ways to kill each other for several decades.

The most efficient way to get NATO troops trained in this new war is to send troops to Ukraine. I suspect this may be one of the reasons that North Korea is sending troops to Ukraine.