except for Hitler has anyone ever attacked Russia first?.....
Off the top of my head, the Kaiser in WWI, Napoleon during the Napoleonic Wars, the Vikings during the Viking Age, the Mongols, and the Japanese(twice).
The Mongols, and the Japanese are the ones Putin should fear... Not literally. Obviously, modern Mongolia, and Japan are nothing to be feared, but an invasion of Russia from the east has always easy. Russia has a lot of trouble getting more forces to the east, and has few forces there now. China could easily take half of Russia. There is a reason Putin is rounding up any Russians being seen as pro-Chinese.
The Viking(Rus) conquered Russia so much they gave it their name.
Napoleon had a huge victory at first, followed by nonstop victories, until his army mostly died from hunger and cold. It was a textbook study in how not to win. To do this, the Russians had to destroy all their own food stores, and housing. This killed ten times more Russians than French. French historians who interviewed Russian serfs a generation later found that the Russians assumed they had lost the war badly.
We all know about Hitler, but what of the Kaiser. During WWI, Germany first had a major victory against Russia, followed by... digging in defensively. The Germans spent most of the rest of the war fighting mostly on the western front. Had they won on the western front, which was close to happening, they would have then pushed further into Russia. But they limited their push into Russia for most of the war. Russia ended up surrendering large amounts of territory in a separate peace.
We often forget Germany won WWI against Russia. They lost the rest of the war, which makes their Russian victory meaningless.
So what do we learn? Russia is hard to invade from the west, but easy to invade from the east. If you do invade Russia, do it slowly like the Vikings, and dig in frequently, like the Germans in WWI. If you reach the outskirts of Moscow at the beginning of Winter, you may think you have won the war, but you have lost.