In the real world, Russia invaded Ukraine in February 2014, responsive to nothing NATO. It was in December 2014 that, in response to Russia invading and annexing Crimea, Ukraine added the goal to become a NATO member to their constitution; Russia's 2014 invasion and annexation was the context for any NATO response. In February 2022, responsive to nothing NATO, Russia invaded Ukraine in an effort to conquer and annex all of Ukraine. Putin and his orcs learned the hard way that that Ukraine was more prepared and not a pushover.
I know this fact is hard for Ukraine-Haters to swallow, but Ukraine is a sovereign country, having full rights to defend its territory - all of it - against foreign invaders. Russia had zero rights to invade and occupy Ukraine, not in 2014, not in 2022. The current war in Ukraine was started because Putin wants to recreate Stalin's empire and believed Ukraine would be a push-over second (remember 2009 and Putin's proxies controlling part of Georgia?) step toward that goal.
So far, Ukraine has:
* Devastated Russia's tank forces
* Killed on the order of 1/4 of a million men and wounded/maimed another 3/4 of a million men,
* Depleted much of Russia's air forces, including, a few days ago, at least 10-15 strategic bombers that are irreplaceable, because long out of production, and damaging others that may not be reparable, for the same reason; shot down many Su-25, Ka-52, and Mi-24 CAS aircraft and helicopters;
* Reduced Russia's Black See Fleet to a couple of SSKs, a corvette or three (if that), and patrol boats; other than patrol craft, the fleet has been forced out of Sevastopol;
* Seriously reduced the usability of Putin's beloved Kerch Bridge, which is critical to supplying Russian troops in Crimea; seriously damaged Russia's oil industry; significantly depleted Russia's SAM system network, making Russia vulnerable to air attacks, at the front lines and hundreds of Km into Russia.
The Balkan countries, Moldova, and Poland, which would have been some of Putin's next targets, have been made much safer by Ukraine frustrating Putin's empire rebuilding.