Leonid Bershidsky recently published an article entitled, Why Russia Stopped at Crimea I found interesting. He states: President Vladimir Putin has boasted that Russia took over Crimea "without a single shot being fired." He goes on to state: "A document published on Monday shed new light on why the annexation was bloodless -- and on the limits of Putin's aggressiveness." In short, the Ukrainian government realized they had a significant ethnic Russian population who would side with Putin, they were broke and they had no military worth mentioning. Declaring war on Russia would be been an excuse for Putin to attack with an almost 1000% assurance Ukraine would fall in a few days or weeks. The Ukraine government wanted to avoid that. I think Putin did also.
He doesn't mind stirring up the internal manure to get his way, but he's not really looking to start a real war with anyone. This article has some interesting insights, but it doesn't really explain why Russia stopped with the Crimea and didn't invade the Ukraine outright. Take what the author says here and add this to it.
Putin doesn't dare over extend himself militarily. Russia is a Hordesland nation with seven gaps they must defend to protect the homeland. A Hordesland nation like Russia has huge wide open spaces with no natural barriers they can stand behind including "infinite flats of Central Eurasia" and the "North European Plain", which runs from "all of western and northern France, Belgium, The Netherlands, southern Scandinavia, northern Germany, and nearly all of Poland" into Russia. As for Central Asia - it's over three thousand miles long with no "meaningful geographical barrier"s.
Russia has seven gaps they must defend.
Russia only has the manpower to man three gaps and two of them revolve around the Ukraine. Russian foreign policy going back to Imperial days has had one goal - create as many buffer zones as possible against invasion. After the fall of the Soviet Union and the separation of so many buffer states they lost two thirds of the former Soviet Union population.
Russia is in it's twilight as a nation. They're breeding themselves out of existence and by 2040 ethnic Russians will be a thin majority or possibly a minority in their own nation. Their population is filled with alcoholics and Russia has a large number people who are HIV positive and afflicted with multi-drug-resistant tuberculosis, and that's "mostly in the fifteen to thirty five age group". That's disastrous for Russia since that's the age group that creates the next generation. They don't have the manpower to man their own army today, and that will decrease for decades.
Moreover, Russia is a low capital producing nation. First, because it's geography works against it, but mostly because the government is stunningly corrupt and incompetent. They've literally destroyed any hope of individual achievement and accomplishment after the fall of the Soviet Union. The corrupt elite are nothing short of organized criminals, and they're in charge of the government, and the economy. And now their one really profitable resource - energy - is selling cheap all over the world and their economy is even in worse shape. Putin can't afford any real wars. He's merely trying to survive!
Having said that - Putin must have a copy of Machiavelli's The Prince because he's been playing chess while Obama has been playing checkers. Putin publically struts and preens giving the impression he's a buffoon, but he's heads and shoulders about Obama in smarts and tactics. Putin isn't looking to expand Russia back into a new Soviet Union. He can't. He knows that. He's just trying to hold back the tide of inevitability. The inevitability that Russia is doomed as a nation right along with the rest of Europe.
Source: Peter Zehan's "The Accidental Superpower: The Next Generation of American Preeminence and the Coming Global Disorder "