Like most news coverage in general, most coverage of the Ukraine situation so far has been convoluted, disjointed, and missing the big picture. As a result, fears about Russia's propensity to invade other countries have been overblown. I'd like to submit an overview of what's really going on, and why Vladimir Putin should be viewed as a shrewd (if amoral) opportunist, not a warmonger.
After the Ukrainian revolution and ouster of then-President Viktor Yanukovych back in February, Russian Foreign Minister Sergey Lavrov declared the Ukrainian revolution a coup d'état, and made it clear that Russia would not recognize the legitimacy of the interim Ukrainian government. Russia then took the opportunity to amass troops near its borders with Ukraine, and to invade and annex Crimea under the pretext that a majority of Crimean citizens would rather be part of Russia than Ukraine. Russia's annexation of Crimea is now widely viewed as a fait accompli: In March, both a Crimean parliamentary vote and a referendum passed by wide margins resulting in an accession treaty absorbing Crimea into the Russian Federation, and even though the legitimacy of these results is disputed, there's no political will to take Crimea back from Russia. The upshot of all this is still uncertain, but to anticipate, I doubt that Russia has ambitions to subvert any other sovereign states.
In light of the strategic significance of Crimea, and recent political developments in Ukraine, one can make sense of Russia's incursion. To elaborate on the strategic significance, Crimean ports along the Black Sea provide quick and easy access to the Mediterranean, the Balkans, the Caucasus, and the Middle East. The Russian Navy's Black Sea Fleet is also based in Crimea, which has been protecting Russian interests and shipping in the region since the 18th century. Moreover, there is reportedly 45-75 trillion cubic meters of natural gas under the Black Sea, and by some estimates 45 trillion cubic meters lying within a 200 mile radius of Crimea's shoreline - which Russia has of course taken along with Crimea. To put this in perspective, all of Russia's proven natural gas reserves as of 2013 are approximately 33 trillion cubic meters, and the entire world's proven reserves are about 187 trillion cubic meters. If the estimates for the Black Sea prove correct, this would put Russia in control of about 42% of all proven natural gas reserves on earth (up from less than 20%). If these estimates prove correct, even half correct, Western sanctions would be little more than a business expense as far as Russia is concerned.
Add to these strategic considerations the political situation unfolding in Ukraine. Former Ukrainian President Yanukovych was favorably disposed toward Russia, and though his government was going to let Exxon and Royal Dutch Shell start extracting some of the natural gas off the Crimean coast, he generally made it easy for Russia to do business in Ukraine, and much of Crimea's natural gas was still up for grabs in terms of extraction rights or trade deals. The revolution that ousted Yanukovych took on a decidedly anti-Russian, pro-European tone, and may have endangered Russia's up-to-then favorable business relations with Ukraine, not to mention its many oil and gas pipelines that run through Ukraine.
With this in mind, I tend to think Putin and his administration took a calculated risk in annexing Crimea. Putin correctly wagered that he could invade Ukraine, take Crimea, and that no one would do anything about it beyond bluster and economic sanctions - in exchange for billions if not trillions of dollars worth of natural gas all at Russia's disposal (the deals Exxon and Royal Dutch Shell had made are now off), along with lucrative new avenues for international shipping and pipelines. This blatant violation of Ukraine's sovereignty is alarming, but there are reasons to believe Russia will not repeat this kind of escapade elsewhere:
These are 1) a lack of incentive and 2) NATO. Post-Soviet Russia has proven itself to be an opportunistic land-grabber (recall the 2008 Georgia-Russia crisis) but only in neighboring countries with no treaty framework ensuring some third party will come to the rescue. Unlike Ukraine and Georgia, all of Russia's European neighbors (with the exception of Belarus and Finland) are members of NATO. President Barack Obama recently reaffirmed the United States' commitment to protecting fellow members of NATO if they come under attack, and even if that is a bluff, it's not one that Russia would attempt to call unless it were insane, or stood to gain something huge.
Which brings us to the other reason, lack of incentive. The natural resources and other advantages to controlling Crimea and the Black Sea aren't present in Russia's other neighbors. There's nothing else like the Ukrainian revolution threatening Russian business interests abroad. There is simply nothing for Russia to gain in Eastern Europe that's worth the risk of triggering NATO retaliation, and its relations with other neighbors are cooperative. Perhaps Finland and Belarus should be concerned, but considering Russia's current stance toward the ongoing unrest in eastern Ukraine, further aggression seems unlikely.
To elaborate, there is a pro-Russian separatist presence in eastern Ukraine that has been in armed conflict for several weeks with the Ukrainian military, and which has allegedly been receiving support from Russian troops stationed near the border. Russia has been in talks with the Western powers and Ukraine to de-escalate the situation, including withdrawing its troops from the border, which it has slowly begun to do over the last two weeks. Moreover, Russia recently submitted a UN resolution calling for the creation of "humanitarian corridors" in eastern Ukraine that would likely include a multinational peacekeeping presence. Upcoming peace talks between Ukrainian President Petro Poroshenko and a special envoy of Putin also augur well.
In sum, following the money is key to understanding this situation and discerning Russia's modus operandi. Russia took Crimea because it was a hugely profitable proposition, with little chance of any blowback beyond sanctions. Russia does not seem to be encouraging the fighting in eastern Ukraine - and there is no Crimea-esque opportunity for another land-grab in sight - making fears of further Russian belligerence largely misplaced.