It bears repeating that an enormous difference between the American response to the Soviet invasion of Afghanistan and the American response to the Russian invasion of Ukraine is that Ukraine...
...is a democracy.
The United States has historically done a pretty bad job at supporting democracies. Since World War II, one can count the democracies the US has supported with military power on one hand.
The Vietnam War is almost a perfect example of Dulles-brother imperial nonsense. I mean, supporting Madame Nhu was some seriously boneheaded and outright evil policy. And again and again, the United States has supported brutal, anti democratic regimes which almost inevitably led to stupendous blowups and self-owns. Supporting Saddam Hussein, then fighting him. Supporting Noriega, and then fighting him. And of course the loathsome crimes of the Khmer Rouge and Pinochet are blood on the hands of the Kissingers of US foreign policy.
But when US foreign policy was to support actual democracies, it's tended to be vastly better. By definition it's better because they're democracies.
And Ukraine is a democracy.
|Alfred Sisley, La neige a Louveciennes|
*Wikimedia Commons contributors, "File:La neige a Louveciennes.JPG," Wikimedia Commons, the free media repository, https://commons.wikimedia.org/w/index.php?title=File:La_neige_a_Louveciennes.JPG&oldid=618754649 (accessed February 1, 2023).