If you're interested in understanding the zeitgeist of the time period vis-a-vis US/Russia relations, the go-to work is Fukuyama's essay, 'The End of History?' from 1989, and the 1992 expansion on it, his book The End of History and the Last Man. These aren't necessarily historical works per se, but rather, capture a popular sentiment at the time, at least politically, that the world's ideological wars were over and a new era had begun: the era of the liberal democracy.

At any rate, a more realpolitik analysis of the situation is a bit less idealistic (or, as some might complain of Fukuyama, ideological). Russia's wealth in the 1970s and early 1980s had grown (or, at least, been maintained) on the backs of strong oil prices. The collapse in 1986 severely strained the Soviet budget, which, along with the ongoing war in Afghanistan, meant that public finances were in pretty dire straits by the time the late 1980s rolled around. Under this lens, the withdrawal from Eastern Europe, and the lack of pushback against the Baltic States, was owed to not only the unwillingness to use the harsh tactics to keep the the Warsaw Pact and more disloyal republics in line, but an inability to do so. Indeed, by late 1990/1991 (http://object.cato.org/sites/cato.org/files/pubs/pdf/fpb008.pdf) the USSR was relying on western loans to maintain their budget. In short, despite the USSR on paper being very strong, it was clear by 1989/1990 that their economy and state was in very poor straights.

However, this started well before the oil shocks. Detente on the part of Gorbachev began almost immediately upon his ascension to power, with the removal of missles in Europe and attempts at building close personal relationships with the leaders of the major NATO states. The USSR began withdrawing from Afghanistan in 1988, and that same year announced that the Soviets would let their eastern sattelites decide their own fate, and almost from the beginning of 1989 this resulted in the overthrow, peacefully or otherwise, of the communist governments in those states one by one.

So, in short, you have the realist view that the USSR was no longer the credible threat it once was, and also the more idyllic view that, thanks to personal efforts of Gorbachev and his government, the USSR was now willing to work with the west rather than against them.