After Ukraine used long-range missiles supplied by the US, Russia fired an intermediate-range missile at Ukraine, a missle that could carry a nuclear warhead.  This was a warning:  if Ukraine continues to fire long-range missiles into Russia, then the next one Russia fires….  Even the NYT has noticed.

UPDATE:  An Australasian philosopher writes:

I gather that it's not just that the ATACMS and Stormshadow missiles fired from Ukraine into Russia are *supplied* by the US (and UK), it's that the actual firing of them requires the *direct participation of US/UK military personnel* – so from the Russian perspective these missiles are actually being fired by the US/UK (albeit with Ukrainian involvement). The Russians have made this point numerous times, including in Putin's latest statement:

http://en.kremlin.ru/events/president/news/75614

As far as I'm aware, this claim hasn't been challenged by the US/UK, so I'm surmising that it's true.

So, to be perfectly clear about this: a Democratic President whose party has just been decisively beaten in a presidential election is using the dying weeks of his administration to fire long-range missiles into the world's most heavily armed nuclear country.  When I think back to the fear we all felt during the second cold war in the early 1980s, I'm astonished that there seems to be so little concern about this: this is genuinely insane ("stupid" doesn't quite cut it!).