There are Russians in Crimea? That is putting it mildly. The Crimea is mostly populated by Russians. The whole eastern part of Ukraine is also heavily populated with Russians. This is a possible excuse for the Russians to organize a “break-away” movement like those in Abkhazia, South Ossetia, and Adjaria in Georgia. In case of conflict, there would be many ethnic Russians who would serve as a pool for recruiting “militia” thugs and sabotaging the Ukrainian defenses.
The Ukrainian army is puny next to Russia’s.
The best policy is to allow time for Ukraine to develop economically and strengthen militarily over the next nine years, and then simply do not renew the lease. Or make it year-by-year based on Russian good behavior, and cooperation in opposing islamic terrorists, instead of Russia’s reflexively helping every whacko, dictatorial or anti-Western regime from North Korea, to Iran, to Zimbabwe.
If this is to be a cold war, the West can win it as we did the first one. A rich and strengthening Poland and Ukraine will look pretty good against the Bialorussian stagnating dictatorship, and will beat that sort all along the line, possibly without even having an all-out war. I mention Bialorus because it is supported by Putin, and represents his way for states bordering Russia. That is a pretty dark and unattractive picture for the people of those areas.
There are also enough Soviet/Russian holdouts in the Ukranian military to cause real trouble should Russia go to war with the Ukraine.
Approximate numbers from Wikipedia.
Russian MBT 22,000
Ukraine MBT 4,000
Russian Interceptors 1,200
Ukraine Interceptors 277
Russia is currently conducting an invasion on its southern border and it also has a large territory to defend. My guess is they could commit 1/2 of their forces to a Ukraine invasion if they wanted. So Russia would only have a 2 to 1 advantage if no one from NATO helped Ukraine. The internal problems caused by the Russian citizens in Ukraine will magnify the Russian 2 to 1 advantage. Russia could take the Ukraine but it will be costly for both sides unless the Russians employed Tactical nukes.