> Russia uses a small nuke on a military target in the middle of nowhere Ukraine. ___________________________. Everyone is firing nukes at eachother.
While all-out international global war isn't guaranteed in this scenario, I don't see why you'd be so confident as to imply that it was very unlikely. For me, the biggest fear in terms of escalating to nuclear war is the time when some nuclear power is eventually beaten in conventional war to the point of utter desperation, and ends up going all out with nukes as a Hail Mary or a "if I can't have it, no one will" move.
Russia uses a strategic nuke in a military move in Ukraine. The rest of Europe, fearing a normalization of strategic nuke use and more widespread uses of these weapons (including outside Ukraine) begin deploying in Ukraine to help push back the Russian forces - especially since Russia showing the willingness to use nukes at all makes them and their military a lot more intimidating and urgently threatening to the rest of Europe than before. Russia perceives this deployment as a direct declaration of war from NATO and invades (one of) the Baltic states to create instability and drive the attention and manpower away from their primary front line. This leads to full war and mobilization in Europe. Russia is eventually pushed back to their original borders, but with what the situation became, Western countries are nervous that not dealing with this once and for all would just be giving Russia a timeout to regroup and re-invade with more nukes at a later point. They press on, and eventually Russia is put in a desperate situation, which leads to them using nukes more broadly against their enemies for one of the reasons I described at the start of this comment. Other nuclear states begin targeting known nuclear launch sites in Russia with strikes of their own, to cripple their launch ability. This is nuclear war.
I'm not saying this scenario is likely, but this is just one attempt at filling in the blank. If you can imagine a future - any future at all where Russia, or North Korea, or India, or Pakistan, or Israel have their existence threatened at any point in the future ever, this is when nuclear war becomes a serious possibility.
While all-out international global war isn't guaranteed in this scenario, I don't see why you'd be so confident as to imply that it was very unlikely. For me, the biggest fear in terms of escalating to nuclear war is the time when some nuclear power is eventually beaten in conventional war to the point of utter desperation, and ends up going all out with nukes as a Hail Mary or a "if I can't have it, no one will" move.
Russia uses a strategic nuke in a military move in Ukraine. The rest of Europe, fearing a normalization of strategic nuke use and more widespread uses of these weapons (including outside Ukraine) begin deploying in Ukraine to help push back the Russian forces - especially since Russia showing the willingness to use nukes at all makes them and their military a lot more intimidating and urgently threatening to the rest of Europe than before. Russia perceives this deployment as a direct declaration of war from NATO and invades (one of) the Baltic states to create instability and drive the attention and manpower away from their primary front line. This leads to full war and mobilization in Europe. Russia is eventually pushed back to their original borders, but with what the situation became, Western countries are nervous that not dealing with this once and for all would just be giving Russia a timeout to regroup and re-invade with more nukes at a later point. They press on, and eventually Russia is put in a desperate situation, which leads to them using nukes more broadly against their enemies for one of the reasons I described at the start of this comment. Other nuclear states begin targeting known nuclear launch sites in Russia with strikes of their own, to cripple their launch ability. This is nuclear war.
I'm not saying this scenario is likely, but this is just one attempt at filling in the blank. If you can imagine a future - any future at all where Russia, or North Korea, or India, or Pakistan, or Israel have their existence threatened at any point in the future ever, this is when nuclear war becomes a serious possibility.