Russia views any country or alliance that is not part of or controlled by Russia as an enemy. The Russians are paranoid and insecure.
Of course they have good reason to be so. Their history is replete with foreign invasions. The Swedes, the French, the Turks, the Poles, the Japanese, the Germans (ESPECIALLY the Germans), etc. have all invaded Mother Russia in just the last 400 years.
Ukraine was much a part of Russia as Scotland has been a part of the UK, up until 1991.
Putin (and much of Russia) misses its Empire and wants it back.
Whether or not Ukraine being reabsorbed into the Motherland is a threat to US national security is a problematic question. Same as Taiwan being reaborbed into China.
Americans were once told we had to fight the Viet Cong in the Mekong Delta so we didn't have to fight them in San Diego.
Americans were once told told we have to fight the Iraqis in Iraq so we didn't have to fight them in Brooklyn.
Lies, on both occasions. At tremendous price.
I don't want to see freedom-loving Ukrainians fall under the Russian boot, of course. And I feel bad for the Uighurs & Tibetans in China.
But enough, already. If Europeans don't care about Russian expansionism on their doorstep, why should Americans?
Narco-terrorism in Latin America is a much more direct threat to US national security. And has been for 20 or more years. If we had spent more attention to events in Mexico, Honduras and El Salvador as we did Iraq, Syria, Iran, Yemen, etc. would we be better off today?