As Russia fortifies its position in Crimea, eastern Ukraine will be the next region to watch in assessing the evolution of the Ukrainian crisis. Eastern Ukraine, like Crimea, is politically oriented toward Russia, in contrast with the more Europe-leaning western and central parts of the country. However, there are significant differences between Crimea and eastern Ukraine that will make a Russian military intervention there unlikely for a number of reasons.
The east, unlike Crimea, is not populated by a majority of ethnic Russians. There is a large population of ethnic Russians in certain parts of the east — and the majority of the population in the eastern regions is Russian-speaking — but there is a big distinction between cultural ties and ethnicity, especially given Russia's military doctrine to protect Russian citizens abroad.
The east is also the industrial and economic heartland of the country. Any military moves there could prove disastrous to the Ukrainian economy and would have spillover effects on the Russian economy. Also, many of Ukraine's key business tycoons, known as oligarchs, hail from the eastern industrial belt. The oligarchs and the political factions they support have been very clear that they want to keep the country together in order to preserve their business interests. And despite the Western orientation of the new government in Kiev, the new administration has tried to reach out to ousted Ukrainian President Viktor Yanukovich's Party of Regions and its oligarch backers to preserve as much of the political and business status quo as possible. Indeed, the acting president has appointed two oligarchs as governors of two key regions in the east, Donetsk and Dnipropetrovsk, showing that they are trying to maintain those links despite initial speculation that the new administration would get overzealous and try to strip the oligarchs of their assets. Military intervention would greatly destabilize such a political arrangement.
Moreover, not all cities in the east are heavily Russia-leaning (though they are very opposed to the Euromaidan movement and its effects in Kiev). Donetsk is the most supportive of Russia, and protests there have been the largest, at around 10,000 people, but cities such as Kharkiv and Dnipropetrovsk are divided among pro-Russians, pro-Europeans and those who are neutral. There have been some calls for autonomy from the current government in cities such as Luhansk and Donetsk, with some protesters calling for a referendum on the cities' future status. Formal plans for these votes have not yet been put in place as with the referendum in Crimea, which is set for March 30. So long as regional governments in the east refuse to recognize the government in Kiev, political debates in the Ukrainian capital are likely to be paralyzed without their participation.
Logistically, a Russian military invasion in the east would be much more complicated than what has happened in Crimea, and it is far from clear that a majority of people would support Russia in the event of an invasion. Russia had a key strategic military presence already in Sevastopol and elsewhere throughout the peninsula, which enabled an easy projection of force into Crimea. Any cross-border moves in the east would constitute an outright invasion and could easily garner too much blowback both domestically and internationally.
Ultimately, it is not in Russia's interest for the east to split off from the rest of Ukraine. Rather, Moscow would like the east to remain a part of any future government structure in Ukraine and serve as a source of leverage for Russia in any Ukrainian government's decision-making. If mainland Ukraine were to split, it could give the country's western and central regions further impetus to seek more concrete alignment with Western blocs such as the European Union and NATO — something that Russia strongly wants to avoid. Especially since the Ukrainian political structure is likely to undergo a decentralization of power that will leave the executive office weaker, maintaining Russian influence over the eastern regions will be increasingly important for Moscow. A territorially unified but internally fractious (and therefore neutralized) Ukraine would be in Russia's interests.
Therefore, the east will be a key component in any negotiations, not only about the evolution of the structure of the Ukrainian government, but also about its foreign policy, which is a battleground for Russia and the West. Eastern Ukraine is largely opposed to the country's integration into the European Union and poses a potential separatist threat to the rest of the country if any government makes serious efforts toward this goal. It is thus a key source of leverage for Russia in shaping the Ukrainian crisis. However, Moscow would prefer to avoid acting militarily or in terms of separatism in eastern Ukraine as it did in Crimea, while maintaining the possibility of such action to strengthen its own negotiating position.