Russia has been the bad guys since the Cold War. So, ever since then we (US Gov't) has pretty much opposed them, not unlike republicans vs democrats. When Putin started the war in Ukraine, politically Biden had to oppose that or look like a pussy. And then he had to back up the rhetoric with war materiel, and hey presto, here we are. Putin should have realized that no US president or European countries could stand by and do nothing while he invaded another country. Hitler taught us that lesson back in the 1930s.
What is the US goal in Ukraine? To save face politically. IMHO, everything else that might be said as far as why we're involved is bullshit.