Syria.... Which to be fair, Russian involvement started before Cheeto Jesus, but the russians were exercising equipment there during the entirety of orange man's rein. Its not like they took 4 years off from foreign interventions, they were just invited that time.
I saw an interview the other day that talked about how Russian brass in recent years came to the conclusion that it was cheaper to rotate their forces in Syria on 3-4 month rotations than it was to adequately train them at home - which in hindsight maybe is indicative of a problem with how they (don't) train at home, at least for the reserve/conscripts.
My dad was 82nd in the late 70s/early 80s and was a TOW/Dragon Missile tech(precursor to the javelin) so I was pretty curious to hear his take, since the Ukrainians are living through what he war gamed many, many times. When asked about NATO forces being used in any sort of active way, he said no, NATO is willing to arm and fight to the very last Ukrainian. The interesting thing to him is going to be the data that comes out of this - Javelins and NLAWs being used side by side, compared to RPGs, effectiveness of stingers, ect. A lot of 1970s/1980s war plans being dusted off right now. Very convenient that the russians were kind enough to send in that vintage equipment too...