The nightmare scenario for a military planner is the so called 3 block war. The first block has offensive kinetic military operations are being carried out, the next block over the Military mission is support to host nations security forces and the next block over from that is a pure humanitarian support mission.
Any military and especially the US military, will be at its best when given a pure conventional offensive force mission. Gulf War 2 was arguably the most effective use of military force in the history of warfare. The assigned purely military mission was simple, defeat Sandam's forces and invade and occupy Iraq.
It all fell apart just like in Afghanistan, when the mission ceased to be a purely military one and morphed into a conflict shaped by amorphous, ever changing and inherently incompatible foreign policy objectives. This IMO reinforced senior military leadership belief that ultimately the only use of military force that really mattered was full on state on state warfare. Therefore it has always resisted involvement in "Nation Building" exercises and concentrated on building conventional forces that can win battles against a peer level adversary. This has been encouraged by the American military industrial complex because it incentivizes the development of advanced and expensive kit.
The fundamental problem is for almost 50 years, since the end of significant American involvement the Vietnam war, the western world has not experienced major sustained conflict. Almost all of the political leadership grew up in this period and they and the general population have lost any appreciation of what military forces actually are designed to do. This I think is ultimately the cause of the current recruiting challenges.
The Ukrainian Military doesn't have any problem recruiting new members. Those recruits aren't joining because they want to help fill sand bags when there is a flood or participate in feel good "peace stabilization operations" they are joining to kill Russians. In a perfect world the political leadership would have a frank and honest conversation to their population about why countries have a military and what it is supposed to do. The chance of that happening are less than zero but I think it would actually improve recruiting.
I think it is deliciously ironic that Putin has singlehandedly done more to reverse the decline in Western Military capability since Hitler, by reminding everyone why countries have military forces.