Historically Russia has been invaded many times by very large forces including:

• Mongols
• Napoleon
• by most of Europe, the UK, the USA and even Australia in 1917-1919;
• Hitler in WWII.

For the entire Cold War both NATO and Warsaw Pact had huge forces threatening to invade one another.

Putin considers NATO expansion closer and closer to Russia to be yet another invasion.

I suggest you read "2048" by Prof Passig in the original Hebrew to get a better understanding of Russia.