You need millions of weapons, millions of tons of ammo, houndreds of thousands radios, same with food, you need to spend in logistics to move your millions of guys, you need trucks, vans, cars, etc. you need fuel, you need spare parts, you need an officer cadre big enough. You need industry. Ukraine or the west doesnt have an industry focused on mass mobilization.
In 216 BC century the romans could send 80 thousands soldiers to die (battle of cannae) and regenerate that number of legions very quickly and send them again. a thousand years laters medival armies couldn't even generate a 10 thousand soldiers most of the time. it's not about time or technology.
