Russia defeats Poland very quickly in a one-on-one war
It's not clear where the notion that Poland has an advantage over Russia in its military equipment, as implied in James A. Nollet’s “Why Russia will Never attack Poland”,comes from, but it is incorrect.
Regardless of how unlikely it is that Poland and Russia would engage in a purely one-on-one total war -- especially given Poland's membership in NATO and protection under Article 5 -- if they did, Poland would lose very decisively, and very quickly. The battle would go along the lines of how the first Gulf War went for the Americans, except this time it would be Russia defeating Poland.
For the hypothetical, we assume Russia attacks Poland, and nobody comes to Poland's defence nor any of Russia's allies assist it.
Russia has nuclear weapons. Poland does not. Consequently, if Russia so desired, it could achieve a victory without any shots by simply demanding the unconditional surrender of Poland under the threat of using Russian strategic nuclear weapons on Polish military and civilian targets. Russia could turn Poland into a glass parking lot without sending any of its forces across the border. Game over for Poland.
But let's assume Russia decides to just employ its conventional forces and fight it out against the Poles without its strategic nuclear arsenal. And we further assume that since Russia is only fighting Poland, and Poland is only fighting Russia, each military can focus all its resources against the opponent.
Russia's cyber forces are very large and arguably some of the most talented. They would start the war, shutting down much of Poland's commercial and military communication capacities and putting its industrial and infrastructure systems out of commission. Russia also turns off the natural gas supply into Poland and begins setting up a naval blockade in the Baltic and North Seas to prevent any supply ships making it to Polish ports.
On airpower alone, Russia could decisively beat Poland in short order. The Polish air force is a joke compared to Russia. In terms of combat aircraft, Poland only has 36 F-16Cs, 31 MiG-29s, and 32 Su-22s. Stack that up against almost 1400 Russian air force combat aircraft that includes 252 MiG-29/35, 135 MiG-31, 277 Su-24, 199 Su-25, 321 Su-27/30, 61 Su-34, 70 Tu-22M, 41 Tu-95, 13 Tu-160, and an active fifth generation Sukhoi PAK FA with 60 more on the way. If the Russian navy joined the air fight, which they certainly would, add in another 14 MiG-29/K, 22 Su-24, 4 Su-25, and 28 Su-30/33. As well, the Russians have the support aircraft and infrastructure to readily carry the air attack in a sustained fashion against a nearby state.
Poland's Su-22s won't last very long, and the MiG-29s -- despite upgrades -- will still be outclassed and greatly outnumbered by the highly advanced Russia counterparts. Even if the Su-22s and MiG-29s managed to take out a couple Russian fighters, that still leaves only a few dozen F-16Cs -- advanced though they are -- up against well over a thousand advanced Russian fighters that are, in many cases, arguably as capable or more capable than the Polish F-16s.
Russia quickly achieves full air superiority over Poland, even with Poland's modestly impressive air defense systems which the Russian military would immediate target and destroy via its massive assortment of highly advanced surface-to-surface and air-to-surface missiles and precision bombs, all with the options of either conventional or tactical nuclear warheads.
Having full air superiority, Russia's large bomber fleet now begins pounding Poland's land-based military targets and removes, in tandem with the Russian navy, what remains of the Polish navy. Without air dominance, Poland's naval targets become sitting ducks in the Baltic for Russia's air assets. Of course, Russia's air-to-surface and surface-to-surface missiles also join the fight early on against Poland's navy, with little resistance coming back towards the Russians.
Within a few weeks to a couple months, Russia has now destroyed the entire Polish air force and all naval assets, most -- if not nearly all -- land-based military hardware, and taken out almost all of Poland's industrial capacity.
At this point, Poland is as defenseless as it was after Germany overran it during WWII. Minimal remaining land-based military hardware, no air force or naval assets to employ, and no electricity or heat or water or sewage. Poland is on its knees, and may yield -- if it is smart -- in an unconditional surrender without an occupying force.
But if not, in comes the Russian army and its capacity to mass more than 1200 combat helicopters and hundreds of transport aircraft, along with upwards of 15,000+ tanks and on the order of 50,000 other ground-based fighting vehicles. Most of Poland's 200 or so combat helicopters would have already been destroyed by missiles and bombs after Russia achieved air superiority, but whatever few remained at this point would be easily dispatched by Russia's combined forces.
The 1000 tanks and couple thousand other ground-based fighting vehicles that Poland had before the war started were massively outnumbered and, in general, outclassed. At some point, quantity has a quality all its own, and Russia's ground forces were well past that point against the Poles before the shooting began. It is irrelevant, though, since most of Poland's ground forces were already destroyed during Russia's air superiority bombing phase.
Thus, Poland may now have some remaining advanced army equipment, but nothing sufficient to do anything other than make a Russian occupation bloodier for both sides -- especially the Polish civilian population that is now in extreme duress after many weeks of no electricity, heat, running water, or functional sewage systems.
Based on the resolve of residual Polish resistance, Russia would have to choose the path forward from this point of victory, but Poland has lost. Poland knows all this would come to pass, which is why it is ramping up military spending and asking for NATO troops and defense systems on its soil.