Probably 9/11. The resulting wars in Afghanistan and Iraq were largely fought by Millennials (with some gen-z nearer to the end, I think—but the initial invasions heavily involved "elder millennials" [EDIT: and x/y "cuspers", especially for Afghanistan], with younger millennials involved in the long middle period). Though Vietnam (among other wars, but it seems most-comparable) was easily within living memory at the time, so I'm not so sure about "once in a lifetime" for the wars. That kind of wildly-successful attack on the US, though—that part might more-or-less qualify.
The US exports military vehicles all over the world, and often is fighting against forces using them, in actual wars (as opposed to the Russia-Ukraine conflict, which is between Russia and Ukraine). We gave $18 billion in military equipment to the Taliban last year. That Ukraine has US military equipment means very little.
The US is providing security assistance to Ukraine, and condemns Russia's advance, but the US is not fighting a war in Ukraine
Naive at best! US has been actively pushing Ukraine against any kind of negotiations. It is not about security here it is about geopolitical power dynamic in EU.
Almost every war everywhere is fighting with AK's somewhere in the mix, often on both sides. Arms is a very dirty industry and many governments play it, often with proxy dealers to allow them to be arms (da-dish) length and offer plausible deniability. Very often, a superpower or country ends up being shot at with the weapons it provided years or decades earlier.