Ukraine is historically a part of the Russian heartland. Going all the way back to Russia’s progenitor state of Kievan Rus of the 10th century, founded by the Rurikid dynasty originating among Swedish vikings, Ukraine has been a part of, or closely connected to, the continuous political entity that we now call “Russia”.
Sure, there were periods where statelets on the contemporary Ukrainian territory were independent from formal Russian control, e.g. the Grand Principality of Kiev was under Lithuania for a century, a suzerain of the Golden Horde for a while, and there were various tribes occupying the contemporary territory in what’s a rather complex history.
However, what’s now Ukraine was really never outside of “Russian” hegemony and culture since the 1000s, and was formally a part of the Russian Empire since the 18th century.
This is not to say that Russia prima facie “has a right” to the territory in any legal or moral sense, my point here is just that they in many ways are intimately connected, and until very recently actually were part of the same political entity.
UKRAINE’S SIGIFICANCE TO RUSSIAN SECURITY
Ukraine became formally independent about 30 years ago, in relation to the dissolution of the USSR. Strategically, Ukraine is indispensable for Russian security.
One aspect of this is the Black Sea region and Crimea, the importance of which was the key reason for Florence Nightingale’s Crimean War of the 1800s.
Sevastopol has been Russia’s predominant warm water port since 1783 (meaning it’s viable year-round) and is the only avenue for power projection through the Mediterranean, affording the only really viable access to the Middle East, as well as the South Atlantic and Indian Ocean.
Crimea also provides Russia with operational capacity in its close vicinity, e.g. for regional troop transportation and protection of its key trade routes passing through the Black Sea, and is vital for Russia’s strategic defence capabilities of the entire southern flank.
Read more here.
While not entirely defenceless, Russia would be very vulnerable if it lost just Crimea.
Read more: What’s really going on in Ukraine