There are a lot of misconceptions that the American media and grandstanding politicians willingly perpetuate.

First, let's start off by saying Russia feels somewhat betrayed by the West. Communism fell and they didn't really get much assistance when they were transitioning to a market economy. Russia got left with the bill for the entire Soviet Union (in the form of external debt, which was considerable), which collapsed in a fairly illegal way when you look at the details.

Putin called the collapse of the Soviet Union a great humanitarian tragedy, which it was. The peoples that comprised the Soviet Union had lived together for as long as the United States has been around, with a few exceptions (some of Central Asia was annexed later). They lived together as part of the Russian Empire before the Soviets came to power. Putin is not lamenting the collapse of communist totalitarianism when he says it was a tragedy, and he's made that clear on numerous occasions. He's lamenting the fact that millions of people found themselves outside "their" countries because of the way people freely moved inside the Soviet borders for centuries. Many of the nations had never been independent countries, like Belarus, Ukraine, Kyrgyzstan and Moldova.

More importantly, though, Russia feels that the US has just done whatever it wants to whenever it wants to, usually to Russia's detriment, and that it sticks its nose into the internal affairs of other countries. Long before the gay rights lecturing we felt entitled to give Russia during the Olympics (I never heard much lecturing to Saudi Arabia, by the way), we tried to tell them that they should sit down to negotiate with jihadist Chechen terrorists (Secretary of State Madeleine Albright made herself hated in Russia by repeating this "demand", which Russia just ignored).

After September 11, Russia shared intelligence, provided supply routes to Afghanistan for US troops to get around having to go through Pakistan, and generally tried to cooperate. However, they opposed the US invasion of Iraq because they had billions of dollars of oil and gas deals with Saddam. They said, "You can't just invade a country because you don't like it. And you're screwing us over." The US ignored Russia.

Not only that, but the US expanded NATO eastwards. If there's one thing that gets Russians paranoid, it's military expansion on their Western border. They have no natural boundaries to stop invaders, and have been invaded many times - the Poles, the Lithuanians, the Swedes, the French, the Germans (twice!) - and nothing really stops the invaders in terms of geography. So Russia's strategy was to have "buffer states" between it and any potential invader. Belarus is a buffer state. Ukraine WAS a buffer state. Georgia WAS a buffer state to the south. The Central Asian republics are buffer states even today.

NATO expanded into the Baltics, allowing US troops to be stationed on Russia's border. Then we announced we were going to put a missile defense shield in Poland and work to add Ukraine and Georgia to NATO. It doesn't matter how paranoid or rational you are, there is a certain point at which you're going to hit back. Russia hit back when Georgia broke a cease-fire from 1993 (Georgia had a civil war upon the collapse of the Soviet Union and both sides agreed to Russian peacekeepers). Russia hit back when Ukraine had its revolution and started talking about rescinding the lease on Russia's main naval base of Sevastopol in Crimea.

So yes, if the US intervened Russia would ratchet that up all the way to nuclear war because it sees that its most vital interests are at stake, namely, its physical defense of its territory. By contrast, the US had no interest whatsoever beyond some vaguely stated notions that really don't make sense when you think about them rationally.