Russia's border with Ukraine is its most vulnerable land border, and Ukraine in NATO means a hostile military alliance with direct access to said border with the ability to militarize it with nuclear capable cruise missiles: https://www.eurasiantimes.com/analysis-us-withdrawal-from-anti-ballistic-missile-treaty-continues-to-haunt-world-20-years-later/

This was not a revanchist land grab nor was it resource motivated, the regions Russia annexed were for the most part already in a several years long civil war against the post Maidan coup government which was carrying out a campaign to ethnically cleanse eastern Ukraine of Russian speakers. By 2022 Ukraine had broken both Minsk treaties in which it promised to cease these ethnic cleansing campaigns. https://orinocotribune.com/former-german-chancellor-merkel-admits-that-minsk-peace-agreements-were-part-of-scheme-for-ukraine-to-buy-time-to-prepare-for-war-with-russia/

As for why the US is fickle with military support, I imagine it's because the US is very aware by now Ukraine will lose this war and simply does not see a reason to continue wasting weaponry when the war has already served its purpose in destroying Russian relations with Europe and putting a newly minted failed state on Russia's border for it to deal with in perpetuity.