You think Putin's decision to invade Ukraine was based on who was or wasn't the US president? *incredulous look*
I think that Putin, like all bullies, is encouraged or constrained by some sort of cost-benefit analysis that measures what he might gain against what he might loose. Bullies tend to oppress those that they can easily victimize.
Whatever else was/is displayed by Trump in all of his actions, comments and interactions with others, there was an ample amount of crazy included.
"Crazy" means that reactions are unpredictable. Bullies prefer predictable outcomes. With the "crazy" of Trump, that was absent.
With a cowardly dementia victim announcing that no resistance would be offered, that predictability increased. Putin's calculation was confirmed by the Afghanistan debacle.
Zelenskyy has proven to be a wild card in all of this, though. When he told Biden he needed weapons, "not a ride", the world, in unison, stopped and said, "WOW! This guy isn't a lying thief!"
Simply by NOT being a lying thief, Zelenskyy showed he is not the standard politician. By not being the worst of us, that is, a lying thief, he was elevated above all of his peers. In the eyes of other lying thieves, "crazy". On this particular point, similar to Trump.
If Trump was still in office or if Putin had understood Zelenskyy's courage and resolve, Putin would not have invaded.
This is not complex stuff.