I think any instance of NATO troops firing on Russian troops will mean a state of war exists between Russia and NATO. The consequences of that could very quickly escalate into a nuclear war because really Russia only has tactical nuclear weapons as a realistic way of hitting back, they aren't just going to sit back while their troops get bombed into the stone age, they'll fire tactical nuclear weapons at every air base launching attacks. That's exactly what any Russian General would be forced to do because it is their only play. Then NATO would either respond in kind, or there'd be further use of nuclear weapons from the Russian side. We would be forced into a nuclear war likely culminating into a strategic exchange. Every General and Commander in NATO knows that, which is why we've only supplied weapons to Ukraine, and why there's been a gradual build up from small arms to tanks to longer range missiles.