I honestly could care less about Ukraine, but I say we should support them as much as we can because the enemy of my enemy is usually my friend. The weaker Russia gets, the better off America becomes.
There is no other country that poses an existential threat to the U.S. China doesn't have enough ICBMs, North Korea doesn't have enough nukes, Iran and other Middle Eastern countries can't project much power outside the region, and all other strong countries are either allies or neutral to the U.S.
And that's really it. Terrorists may pose a threat, but ISIS, Al-Qaeda, and the Taliban combined can't turn the entire country into a radioactive wasteland, and neither can border caravans, drag queens, or men competing as women. There's really no other force we should fear.
If Russia invades Mongolia, arm Mongolia. If Mongolia invades Ukraine for some bizarre reason (they're thousands of miles apart), or vice-versa, do nothing.