Some of this is ingrained in the Russian mindset. After centuries of being continually invaded, including most recently a very brutal Nazi invasion, most Russians are just conditioned to expect the next one.
That being said....what makes you guys think that WE WOULD BE INSANE ENOUGH TO INVADE RUSSIA? You guys being a top-level nuclear power and all? Witness how our limp-wristed Left freaked out when we invaded Iraq, a pipsqueak country, with historically low casualties. You think they’re gonna sit by calmly for World War III? And besides, don’t we have enough problems already? We’ve got enough to deal without adding rampant alcoholism and failed agriculture to the mix, thank you.
Including centuries of Russian invasions and annexations of foreign territory. Russia is almost twice the size of the next largest country on earth because it has been an aggressive acquirer, by force, of neighboring lands for most of its existence - a habit that has tapered off a little in the face of the nuclear age.
Russia’s ally turned on them after Russia had already invaded Finland and Poland.
Right now Russia is invading Ukraine.
Our defenses must be maintained to defend ourselves from Russian aggression.
"most recently" being 74 years ago, and being immediately preceded by Russian/Soviet invasions of the Baltic states, Poland, and Finland.
On the other hand, Russia under various regimes invaded Poland in 1654 and 1792, Finland in 1808 and Sweden in 1809, Moldavia in 1853, the Ottoman Empire in 1855, Bulgaria in 1877, the Ottomans again in 1878, Tibet in 1904, the Ottoman Empire in 1915, Poland and Anatolia in 1920, Manchuria, Finland, Poland, and the Baltic States in 1939, Indochina in 1941, Germany, Bulgaria, Albania and Yugoslavia in 1944, Slovakia, Croatia, and Manchuria in 1945, Hungary in 1956, Czechoslovakia in 1968, Afghanistan in 1969, Georgia in 2008, and Ukraine in 2014.
Maybe the Russkies should stop being so paranoid.