The most obvious example I can think of – and I fully defend the analogy in this case as long as it's understood in its limited context – would be the Nazi use of ethnic German fifth columnists in places like Czechia and Poland.

It seems pretty close: you have a historical major power which feels itself downtrodden and cut down to less than its rightful size. You have an adjacent territory, once belonging to that powerful state, and with many minority citizens of that power's ethnic group; but the territory belongs to a young and much weaker state, founded as the home of a nationality which the powerful state claims to regard as illegitimate, fake, and even a foreign conspiracy against its interests. The powerful state evidently wants to seize the territory or perhaps even the entire smaller state, but is reluctant to do so openly because it probably can't get away with naked aggression in the face of more powerful foreign guarantors. So it fosters an ethnic conflict through the use of proxies and military infiltrators in civilian clothes, uses the resulting conflict for propaganda claims that the smaller state is vengefully persecuting oppressed minorities, and then sends in armies to "protect" them.

Nazi analogies are overused and often hyperbolic; and we should acknowledge that Russia's strategic position is on the whole much less threatening to the overall world order than was Germany's in the late 1930s. But yeah, if you want an analogy, you're not going to find a closer one.

The russians did utilize partisans and other irregular forces for sabotage, recon, and capture. For example, Operation Bagration (the retaking of ukraine, poland etc from germany) involved partisans who, with varying levels of success, sabotaged railroads, set ambushes and passed intelligence. They essentially helped prepare the way for the main force to make assaults easier, but after the area of operations was secured many partisan organizations found themselves disbanded.

David Glantz's "Soviet Military Deception in the Second World War" and Paul Adair's "Hitler's Greatest Defeat: the Collapse of Army Group Center" are a good foothold on the subject.

The use of unidentified Russian forces claiming to be spontaneous popular militias in Crimea resembles very loosely Morocco's Green March and annexation of Western Sahara in 1975.

Western Sahara was a Spanish colony, rich in fishing banks and phosphate mines, that was never ruled by Morocco but was claimed by this country under very flimsy arguments (the main one being a letter signed by some religious leaders in Western Sahara speaking of the Sultan of Morocco as their sovereign, a century earlier, something like that). The territory had a local insurgency, the Polisario, which aimed to establish a republic independent of both Spain and Morocco, and the UN (and the US) was pressuring Spain to leave the territory as part of decolonization.

So coinciding with the death of Francisco Franco and the special situation it created in Spain itself, Morocco organized 350,000 unarmed civilians to just walk over the border and annex Western Sahara to Morocco before it was given independence. These civilians were watched over by 20,000 Moroccan troops who did not cross the border themselves, so it wasn't an act of war.

Spain's army in the Sahara was put into an odd position. If it fired on the unarmed civilians Spain would be condemned by the rest of the world (which it already was because of its regime). If it attacked the Moroccan troops which were leading and supplying the civilians but not actually engaging in military operations in Western Sahara, it would be an aggressor and likewise condemned. So the Spanish government simply told its soldiers to do nothing at all, or even to clear the border of mines to avoid an incident, and later recalled its troops and handed over formal control of Western Sahara to Morocco and Mauritania (Mauritania initially occupied the lower third of Western Sahara, but abandoned nearly all claims and withdrawed in 1979).

Nowadays, Morocco administers Western Sahara and considers it part of its territory. No other country formally recognizes it, but any deal over Saharian mines and waters is negotiated with Morocco.

When Socialist Federal Republic of Yugoslavia was breaking up a similar military strategy was seen by the then mostly Serb and leftovers of Yugoslavian Peoples Army.

Milosevic for example set up paramilitaries, which is what the Eastern Ukranian pro-russian protestors are, in foreign countries and using them to further political agenda of Serbia, and maintaining pressure that the main and real force of JNA would step into action and "protect" the civilians and paramilitaries.

Read up on the war in Croatia from 1991 I believe it was, there is also a region called Krajina (which means the same as Ukraine) where the war began, so its similarities not only in strategy but also in name, fun!

This question has been removed because it violates the rule on poll-type questions. These poll-type questions do not lend themselves to answers with a firm foundation in sources and research, and the resulting threads usually turn into monsters with enormous speculation and little focused discussion. At first blush, it is a worthy question, but it leaves the door open for all sorts of answers about historical conflicts that superficially resemble this ongoing conflict. If you'd like, you may PM /u/caffarelli to have your question considered for an upcoming Tuesday Trivia thread.

Edit- also, I believe this is a violation of the 20-year rule. Again, this is because you are asking for past examples that are analogous to something that is happening right now. That raises the question of exactly what is going on, or what is the Russian or Ukrainian strategy, leading directly to discussion of current events.