By joining NATO and then hosting NATO forces on Ukraine territory. Basically.
It would be similar to China building bases and hosting forces near the U.S. mainland. Very provocative.
In fact, China is planning to refurbish an airbase on Kanton, an island that is 3,000 km from Hawaii, and the U.S. is concerned: https://www.reuters.com/article/us-china-kiribati-exclusive/china-plans-to-revive-strategic-pacific-airstrip-kiribati-lawmaker-says-idUSKBN2CM0IZ
Historically, the plains of Ukraine have been used repeatedly by Western europeans to invade Russia, including Napoleon and Hitler, so that concerns them as well.
Rod Dreher, who writes at The American Conservative, has written a couple of articles on the underlying issues: https://www.theamericanconservative.com/dreher/
Keep in mind, this isn't to justify or excuse what Putin did, but it does go a ways to understanding why it might have happened.
@Kamaji It explains the historic base of their paranoia,but even historically that didn't really mean anything because the USSR is just too freaking big for anyone to occupy. Everybody that has ever tried since the Vikings moved in and took over has failed and left nothing behind but offspring and bones.
As for Europe taking over Russia goes,that is hysterically funny. None of them can even control their own pissy little countries,never mind a nation as big and as diverse as Russia.
The truth is not even the Soviets never really controlled Russia once you left the cities. They just basically left the rural people that didn't live in Ukraine alone and the rural people left them alone. Hell,other than occasionally shopping for a gun or some other tool,the rural people in the USSR had no need to come into contact with the Soviets unless maybe they were fur traders or merchants.
And there is some question about how thorough their control was over even the civilian population of Moscow and St.Petersburg. The city governments were true believers and fanatics for the most part,but the typical Soviet Subject just played along to get along.
Much like today's Americans.
