Russia has further ambitions; defending Ukraine isn't just about reclaiming Ukrainian territory that's been illegally occupied, it's also about preventing further incursions into other territories
No, Russia has been warning NATO to stop expanding for decades, so you can't say NATO expansion is an excuse for pursuing territorial ambitions.
Russian expansion Westward is aimed at protecting itself, having been invaded by Napoleon and Germany in recent centuries. The Soviet Union, which NATO was designed to defend against, no longer exists. So why create a power imbalance that turns NATO into a threat? Like it or not, balance of power
is a thing. The OSCE says so, stating that a state should not increase its security at the expense of another's security. An imbalance of power creates a 'security dilemma'. This is where if one state increases its military strength, even if only for defensive purposes, a neighbouring state may see that as a threat, and take countermeasures, and so a cycle of escalation begins that can lead to war.
NATO expansion has driven Russia and China closer together - exactly what the US was trying to avoid, according to an article by Wess Mitchell in 2018, where he discusses non-military ways to lower the risk of the US having to fight a war on both Western and Eastern borders of Asia at the same time. He also says that it may be necessary to "firmly slam the door to Russian Westward expansion" by inflicting a military defeat upon it in Ukraine, to teach it a lesson. Well, that military defeat is looking less and less likely.
So it would be better if the US leaves NATO, or limits its role within the bloc to responding to nuclear attack on a member state.
