If you asking the TRUE aggressor, then NATO is certainly the...

If you asking the TRUE aggressor, then NATO is certainly the most successful aggressor since WWII.
Has Ukraine invaded Russian territory? Good Question.
No on the Westphalia level, Yes on the geopolitical history level. Some of the territories transferred to Ukraine were based on assumptions in USSR era. When these assumptions are breached, the transfer agreement is void, in geopolitical terms. The inexperienced Russia/Ukraine/Belarussia leaders were inexperienced on that level when they partitioned USSR. In contrast, British government transferred Christmas Island from Singapore to Australia on the Eve of its independence. That is precaution. That is experience. That is something the retarded east Slavs can never compete with Anglo-Saxons.