Russia can impose sanctions - closing the border, granting refuge to people from Crimea and the eastern Ukraine, raise the price of gas supplied to Ukraine and withdrawing the recent economic assistance package.
Russia’s leverage can bring the new regime in Kiev to its knees. The country is but a short step away from insolvency. Its new authorities have to work with Russia. Otherwise the consequences could be very unpleasant.
Ukraine can shut the gas pipelines transiting the country, and remove the Russians from Sevastopol in return.