It goes a bit further than that. There’s also treaties from the dissolution of the Soviet Union involved with the denuclearization of Ukraine. Something the Ukrainians only agreed to if both Russia and the US supported their territorial independence. Russia violated it, but the US wasn’t exactly in a position to respond outside of sanctions and military funding and supplies to the Ukraine. The treaty also wasn’t as strict as NATO.
A lot of this is Putin trying to play tough to the domestic scene as he thought the Obama administration would shrug its shoulders and do nothing when the Crimea was yoinked. The Obama administration instead responded by directly targeting the oligarchy around Putin and brought the rest of Europe in on it, something that made the Russian economic situation (which was already not very good) even more precarious. Russian military modernization was brought to a screeching halt as all their planned purchases of various western military and industrial tech and equipment evaporated. They’ve had to go through the Turks for stuff. The fact is that Putin’s policy for Ukraine failed spectacularly. Russian military casualties were far in excess of what they wanted, demoralized the military as the Ukrainians fought hard and well. Russia’s useful idiots also nearly got NATO directly involved with the Ukrainian Civil War as they shot down an airliner full of Dutch nationals. The Dutch government was extremely close to calling for full article five involvement.
