We can't invoke Article 5 on Ukraine since they're not in it. We also couldn't even invoke it even if our forces were stationed there and attacked, because language of the treaty is very specific - only protects military forces on national territory and naval assets in the North Atlantic (more or less - there are some additional provisions such as French forces in Algeria, etc. but it wouldn't apply to forces in Ukraine)
But yeah if I was POTUS right now, I would give Putin ultimatum on what I think would be acceptable peace. Mostly freezing conflict on roughly current lines - as much as a bitter pill that is to swallow for Ukraine. Otherwise I would allow airstrikes on any Russian military assets currently on Ukranian territory. UAF would then be free to mop up. Basically the threat would be, knock it off or you end up losing anything you gained.