None of the nuclear powers can be invaded. Intervening in Ukraine is one thing and invading Russia is another. If the UK and US send their troops to Ukraine, it will not necessarily start the WW3. On the other hand, if they send their troops to invade Russia, it will definitely signal the start of WW3 and MAD. Russia will definitely use some of its 10,000 nuclear war heads and half the world will turn to radio-active waste land.
perfectly agreed. i am sure that nobody dares to start a war with Russia because russia has got a lot of nuclear weapons which sound the death knell of humanity. For that reason, none of the western countries will invade Russia for the mankind's sake.