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There's a substantial difference between modern NATO-style armies and how most of their predecessor armies (other than troops used in colonies) did it in the 1880...1945 period.
Armies of old developed an officer corps (and some of them also a senior non-commissioned officer corps) to have qualified leadership for a huge mobilised army ready. This included some briefly-trained reserve officers and reserve junior NCOs, which usually had a good education (and thus social) background.
Enlisted personnel was trained and then discharged after at most three years, often two years. The idea wasn't to have super-competent enlisted personnel. The idea was to be able to mobilise millions for the next great war. Peacetime strength of the army was of much lesser importance than wartime (mobilised) strength.
I should more often use images to break up text walls again
The training shortcomings of the time up to 1916 weren't rooted in this; they were rooted in a lack of understanding how best to tackle tactical problems. It was later shown over and over again that you can train young men to become highly proficient enlisted personnel in less than two years. In fact, in times past many junior officers had less military experience than that and were sent to lead men in battle, often even without anyone more experienced supporting them.
Modern armed forces are very much concerned with peacetime personnel strength. But what's the point of having enlisted man on active duty for more than two years? His learning curve has flattened already, he's little more than a prop for a handful of junior leaders to gain experience in leadership and management. Couldn't those learn their trade also by training another class of enlisted personnel? Two men serving two years as enlisted personnel each gives about the same experience to the junior leaders as one man serving four years.And they are about twice as much enlisted personnel in wartime.
To recruit two young men for each two years is harder than to recruit one for two years and then bribe him to re-enlist, of course.
Our modern armed forces have thus been formed by the socio-economic background, just as knights were a product of their feudal time, for example. I'm jut a bit irritated that people seem to believe that this way of running an army is superior, "professional", possibly even "elite". It's a recipe for having too small trained reserves for when you need to have a numerically strong army.
NATO can keep following this recipe as long as it doesn't fight a land war against India or China. We are outnumbering the peacetime Russian armed forces roughly 2:1 with European NATO alone. Recent events have show that Russia cannot mobilise any better than we could. So it's safe to keep following this path in NATO, albeit it's guaranteed to be cost-inefficient.
What about countries that are not part of such a large (and effective, unlike CSTO) alliance? They should abstain from some follies like miniature balanced forces (such as having a navy just because you have a coastline, or running a single squadron of Mach 2 strike fighters). They should also absolutely abstain from the "professional" army model.
It takes more than just a budget to motivate enough young men (and women) to volunteer for a relatively brief military service, but they would do it if it's a career boost rather than a career speed bump. There's no need for conscription in most countries. Yet for deterrence and defence there's a need for quantity. Look at Ukraine and Russia; both need plenty infantry for the oh-so usual infantry tasks. The terrain is mostly open fields, but the demand for infantry is still extremely high. Russia in particular did not prepare, and can apparently not mobilise infantry in quantity with decent equipment and at least modest (six months) training.
Georgia, Azerbaijan, Armenia, Iran, Vietnam, South Korea, Pakistan and Philippines need substantial infantry reserves. The NATO model of land forces would be a folly for them.
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