*wing-shrugs* I kind of knew this was coming even before Crimea. The poisoning of Victor Yuschenko was my first indicator, and that was back in the 2000's. I was not certain, at the time, that it would come to the Russian military actually sending troops into Ukraine, and at the time. At the time, it looked like their style ran more along the lines of destabilizing their neighbors and putting their thumb on the scale in favor of anyone that is willing to act as their boot-licking patsy.
Ukraine's "Orange Revolution," which was a story that I followed with a degree of interest at the time, was a bold move, and this war is going to be a test of whether the Ukrainian people are willing to shed blood for the independence that they originally sought in that "bloodless revolution." It would do my heart good to discover if they could partly hold off a military invasion.
I think it is nevertheless unavoidable that the rest of Europe is going to have to intervene if they are going to deter Putin from marching on Lithuania. He is basically holding the world at gunpoint with threats of nuclear escalation, and his attitude is one of a man that is determined to imperialistically goose-step to his own doom.
Yes, I do speak of his doom as something that is inevitable. He has gone down a dangerous narcissistic rabbithole, along with the lunatic that surround him. If Ukraine rolls over without very much of a fight and if the other western governments remain reticent to retaliate, then it is only going to fuel both his overconfidence and his sense of self-justification. Military conquest, for national leaders, is a lot like crack cocaine. They find it easy to knock down peacetime governments, especially ones in countries that host a large number of likely defectors and whose national leadership really has only a weak commitment to independence. Before those first successful conquests, they might still be dimly aware that the world in general is not just one large Munich, but just like a cocaine-addict's obsession with revisiting that first great high, they only become more bold in their risk-taking.
For example, it is logistically feasible for Putin to march into Lithuania and Latvia, and because of the fact that these countries were once a part of the USSR, it would fit in with his fantasy that he is going to revisit the full glory of the Soviet era. Even as he marches on Ukraine, he might deny even to himself and even to his closest advisors that he would ever want to invade Lithuania or Latvia, he would not be taking into account the political circumstances that a Russian presence in Ukraine would create. He is no Kasparov, and he thinks only three moves ahead, forgetting about the fourth move where greed springs its deadly trap.
If Europe and the United States were having the natural response of feeling hostile toward Russia following such a conquest, then that hostility would offer him a pretext for entering Lithuania and possibly Latvia. Given his strong alliance with Belarus, it would serve his strategic interests to handicap western Europe's ability to position troops in a location where they could easily march into Ukraine to assist anybody left there that is still willing to risk their necks defending Ukraine's independence. In his head, it would NOT be out of a desire to have Lithuania, but it would be out of a desire to stop Lithuania from being used to deprive him of the sought-after prize of a fully Russified Ukraine, but once he had set his sights on a second prize, it would become an obsession.
Putin's only real hope of stopping this process would be to let go of any Ukrainian territories that actively resisted being dragged into the Russian Federation. This would serve his interests in very important respects. It would show the world that he is not going to subject an unwilling province to becoming a part of Russian territory, for one thing. Furthermore, he would thereby avoid the scenario where sustained and bloody resistance against him, in Ukraine, could put him at serious risk of provoking the sympathy of western Europe. He would have time to consolidate valuable territory that is host to people that had strong sympathy toward his government. This would give him a large degree of control over the northern Black Sea region.
I suspect that Putin just might fall into the former trap, though. The reason why I think so is that he has already denied the legitimacy of Ukrainian independence from Russia. He has made up his mind to treat the independent government of Ukraine as an insurgency. He has thereby put himself into a position where he almost has no choice except to pursue the total annexation of Ukraine into Russia. If he were to stop short of such annexation, then this would erode the illusion he has created, among his followers, that he is incapable of being wrong and incapable of being beaten. His style of leadership is not amenable to such phrasing as, "I realize that I said that Ukraine is not a real country, but the situation is not really what I represented it as. We might have to make compromises."
In other words, the trap of territorial greed is already starting to close around his ankle. It was far too easy for him to take Crimea, and it was far too easy for him to take control of substantial territory in eastern Ukraine. His closest followers are not going to be at peace with him giving up on what looks like an easy victory over the rest of Ukraine, at least not after he had already publicly denied the political validity of Ukraine.
While there might be plenty of spots of sustained resistance in Ukraine, the fact of the matter is that Ukraine was never really militarily ready to put up much of a fight, so rolling Russian tanks all the way across the country should be childishly easy. Such a rapid expansion of Russia's territorial borders will ultimately seal his fate. The escalation of tensions between Russia and the rest of Europe will make it hard for him to justify leaving Lithuania or any other surrounding territory as a vector of attack.
In other words, he's a dead man walking.