Invasion was successful because they successfully moved their armies into Ukraine and they are still there. Invasion would fail if they didn't manage to do that.
Rational people still, in 2026, operate under a notion that Putin has some definitive target. Well, he does not, the invasion IS a target. The process is a target. So while the war goes on, the whole operation is a success to him personally.
Same with hypothetical NATO invasion. It's target won't be neither victory over the alliance, nor any gains in territory or cities. The point would be to create a permanent ongoing invasion(s) inside NATO member's borders and sustain it.
Rational people still, in 2026, operate under a notion that Putin has some definitive target. Well, he does not, the invasion IS a target. The process is a target. So while the war goes on, the whole operation is a success to him personally.
Same with hypothetical NATO invasion. It's target won't be neither victory over the alliance, nor any gains in territory or cities. The point would be to create a permanent ongoing invasion(s) inside NATO member's borders and sustain it.