> have precisely zero interest in preemptively invading
And yet no single (super)power in the world would accept this assurance and allow adversaries to take positions at their borders. The US didn't allow Russian missiles in Cuba, and most definitely wouldn't allow China to set up base there today despite any assurances that China would never preemptively invade the US.
There may be many reasons behind this war that we'll never know but the one you don't have to guess. It's been tested and proven accurate over our entire history.
Small countries fear invasion. Superpowers know that they're too large to be outright invaded, but are much more concerned that somebody's nukes (or whatever missiles) can reach their capital before their anti-missiles gadgets can intercept them. It's like having a gun to your head. When you attempt to point a gun to a mafia boss' head, they will remove the gun and neutralize you.
And yet no single (super)power in the world would accept this assurance and allow adversaries to take positions at their borders. The US didn't allow Russian missiles in Cuba, and most definitely wouldn't allow China to set up base there today despite any assurances that China would never preemptively invade the US.
There may be many reasons behind this war that we'll never know but the one you don't have to guess. It's been tested and proven accurate over our entire history.