In my view three things: 1. NATO Article 5 invocation, i.e. an armed attack on a NATO nation 2. or a disaster so terrible that the security of Europe requires

In my view three things: 1. NATO Article 5 invocation, i.e. an armed attack on a NATO nation 2. or a disaster so terrible that the security of Europe requires that NATO enter the war i.e. a nuclear reactor meltdown< 3. a NATO ally such as Poland deciding to enter the war on Ukraine's side. An NATO Article 5 invocation is the most likely. Russia is taking heavily loses due to NATO arms shipments to Ukraine. To reduce these arms shipments Russia must either bomb the shipments near the border or control the border. Both of these could result in an accidental attack on Poland. Russian bombers often have difficulty navigating and use unguided bombs. If they stray into Polish airspace they make be shot down. If Poland decides that war with Russia will happen sooner or later, they make choose to enter the war on Ukraine's side sooner when Russia is least prepared.