***START Spoilers/logistics rant*****
I used to play an RPG
Twilight:2000 that had an almost ASB optimistic perception of how much modern military gear could survive a slow-motion escalating tactical-to-strategic nuclear exchange over a year.
LSS- cool stuff (T-90's and M1E1's) existed and "worked" three years after the nukes flew and wiped out the logistical base b/c Rule of Cool and folks will "improvise".
I work with very complex gear (hydrolectric plant, pumps, motors, gas feed systems, and other stuff run by SCADA) with a perfectly functional supply chain, skilled people doing their jobs professionally and stuff still breaks down.
W/o available parts and goodies, proper lubes, filters, and other facilitating supplies, stuff breaks down really, shockingly fast under normal usage, much less combat stresses.
I can improvise simple parts that might work 80% as well as a manufactured part with the same materials, from OEM drawings and specs, b/c I'm a mechanic and NOT a mechanical engineer and most likely not the CNC lathes and other sophisticated gear they use to make the parts.
With improvised materials, it becomes a devolutionary spiral 60% of 60% of 60% and you've got dysfunctional junk that needs 3-10X the mechanical TLC to keep running for 1/3 to 10% of the original uptime.
Behind the numbers, it means Sh*t works less and less well until there's nothing even close to useful going on.
***END Spoilers/Logistics rant***
Seriously it depends on who's fighting with what, with how many troops.
A mortally-wounded US or USSR that suffered 95+% casualties of people and key assets- power plants, refineries, factories, universities, and logistical networks, not to mention mechanized agriculture, due to disruption of the supply chain w no fertilizer, seed, or pesticides, and it gets medieval really quick even IF some flavor of functional govt survives.
NOBODY's thinking of some revanchist campaign when they won't even have horses to work with.
Consult
Threads for a look at a post-strategic nuclear exchange world.
IF things stayed conventional, it depends on who's lucky and prepared while the first-line gear and crews are available and stocks of the wonder weapons exist.
After D+30, NATO and Warpac are thumb-wrestling.
The big problem with those OMG's the Pact used was they were all teeth, no tail. Baaically they're a "win or die" offensive formation.
No repair facilties, inadequate medical facilities, and the thought the Red Army put into supplying them would make a US QM Corps officer die with laughter.
Against determined resistance, they'd get beat the ^&*( up, and effectiveness would degrade to armed mob within seven days as they went from Cat A to Cat C quality troops and gear. Sure they'd kill a lot of people and wreck some stuff.
Germany would be a dead zone if both NATO and Pact loosed the nukes and CBW agents on hand ca 1985.
No question the NATO troops would be in sad shape themselves by that point as well, but with they could hold off the Pact from making any decisive gains IF some flavor of C3I exists and everyone decides to keep fighting.
YMMDV but I see a WWI-style Cambrai going on as the various parties lose the ability to send and equip troops for at least a year sorting out the post-apocalyptic chaos.
You could end up with the Hundred years War of maruaders ravaging the irradiated, toxic landscape for decades until s/b gets enough people and gear together to smash all resistance and impose peace.