The invasion of Ukraine started in 2014, back when Ukraine had practically no military. Putin escalated this invasion due to the fact Ukraine had been building up its army in response to key parts of its country being occupied by a foreign invader.

If Ukraine had a large military in 2014, I very much doubt Putin would have invaded Crimea nor would there be Russian troops in the Donbas.

Since my point wasn't obvious, let me explain. Ukraine had a very weak military. If it was stronger, Putin would never have invaded.

Strength is multidimensional. Power projection and defense entail different capabilities. It's not like a "strong military" is something you order online, it takes an economic and demographic base which Ukraine couldn't support. Maybe a better argument is that they shouldn't have denuclearized -- for defense you don't need a military, you just need MAD.

ah I see now that I mistook the side of the conflict you were making reference to.