ISTM the critical firebreak issue will be whether the war resembles Korea, wherein (loosely speaking) the US & China fought a proxy war entirely on their proxy’s territories sparing each others’ homelands and thereby ensuring each side had massive strategic depth to retreat / withdraw into if necessary. Which depth the other side would / did / could be relied upon to respect as a sanctuary.
Versus what we all expected in a NATO / Warsaw Pact conflict: Every NATO country except the US would be attacked or threatened at H-hour with long range weapons of some kind. Likewise NATO plans would counter-attack the entire depth of the WP excepting the SU proper. As well, every target within the SU & US proper would become legit game if the other side began to lose the tactical situation too badly.
IOW …
From an escalation perspective today there’s a world of difference between NATO (whether US or German or Polish) & Russian army forces in direct armor / infantry combat on Estonian territory vs. the same thing happening *plus *non-nuclear cruise missile strikes on NATO airbases in Germany / Poland and Russian bases in Russia / Belarus.
We (i.e. both sides) *might *be able to keep the lid on the former and end it some place and in some manner resembling Korea in 1953. Once anybody crosses the firebreak into the latter scenario it’s hard to see where it stops before somebody fires an SLBM.
As others have noted, another difference from the Fulda Gap or WWII days is the relatively thin magazines these days. Either side could be running out of advanced weapons (PGMs, etc.) pretty early in the war. How they react to that eventuality and how the other side responds will be key.
In the early-mid Cold War days NATO recognized it was so out-gunned by count of men & tanks that it expected to run out of those before it ran out of ground to give. At which point the choices were surrender the continent or use the tactical nukes.
“Escalate now or lose now” has been the unfortunate plight of overmatched fighters since Og & Grog first faced off armed with a stick and a rock. Even if you’re likely to lose either way, choosing not to escalate is a *very *gutsy call. Faced with this situation, most decision-makers in history have chosen to escalate. Some won; most didn’t. Either way the collateral damage increased greatly.