I am biased towards reading German sources and secondarily English sources when it comes to history, military affairs, politics et cetera - simply because I am German. I can read French, Spanish, Dutch and Swedish a little, but it's tiresome and I don't understand most details in such texts.
This means I am somewhat above global average under impression of the phenomenon that pre-war allies or early wartime allies switch sides. Sometimes I do some side blows on Italy for this reason. No German should consider Italy as allied if history was a reliable predictor of the future.
Well, this topic is finally a bit in fashion these days, because (suspected) Russian subversion efforts are perceived as more successful than (perceived, and in part real) Soviet subversion efforts of the Cold War era. The idea that certain countries would not want to honour their article 5 North Atlantic Treaty obligations* has entered the realm of debatable topics.
The practical consequences of this are very messy. Plans would need to be drawn up for collective defence that treat all unreliable allies' armed forces as uncertain luxury.
It gets even worse if one country was under suspicion of switching sides in the event of war, kind of what Italy did in 1914/15. This is a most unreliable case, but also a terrible one. Potentially hostile mobilised armed forces worth 150,000 personnel could not be tolerated in NATO's rear. To properly keep them in check would require more than 150,000 personnel for the entire duration of the conflict - an unacceptable diversion of military strength for a weakened alliance. A coup de main (quick invasion and disarmament, from multiple directions) with or without a later occupation by non-frontline forces would be thinkable**. This would require maybe twice as many forces, but only so for few weeks. It could be a prelude to a counteroffensive to liberate territory occupied by Russia.
To eliminate the armed forces of an ally that's perceived as a threat is a ridiculous idea in our time, and would be highly illegal. Still, I have little doubt it would be considered as a scenario once the clouds darkened enough to make the now still ridiculous idea of a NATO-Russia war reality.
To eliminate the armed forces of an ally that's perceived as a threat is a ridiculous idea in our time, and would be highly illegal. Still, I have little doubt it would be considered as a scenario once the clouds darkened enough to make the now still ridiculous idea of a NATO-Russia war reality.
It gets even more messy if one thinks of the potential for subversion inside Russia. Russia is still a multi-ethnic country and could face sponsored insurgencies in many places once its armed forces are busy elsewhere. This could hugely add to the misery caused by a NATO-Russia war.
Another diversion effort against Russia could happen at secondary fronts. The Caucasus region as a whole could descend into war with Georgia taking Abchasia and South Ossetia and Armenia fighting against Azerbaijan again.
I like to think that a conflict between great powers would either be fought through proxies with little direct involvement of the great powers' forces or as a short & quick limited war designed to prevent escalation beyond a region.
Sadly, there's much potential for escalation. An unlikely nuclear escalation isn't the only terrible scenario.
S O
P.S.: I know it would have been more politically correct to talk of red country, blue alliance, orange country et cetera. I wasn't in the mood for such chiffres.
*: Like the U.S. routinely doesn't honour its article 1 North Atlantic Treaty obligations.
**: The most intelligent reaction by the government under attack would be to stand down its forces, agree to disarmament in return for a withdrawal of all invaders' forces within a few weeks. To surrender one's arms can sometimes serve the own nation's sovereignty and security better than to fight with them.
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**: The most intelligent reaction by the government under attack would be to stand down its forces, agree to disarmament in return for a withdrawal of all invaders' forces within a few weeks. To surrender one's arms can sometimes serve the own nation's sovereignty and security better than to fight with them.

