For many Americans, the USSR was a Marxist State, whose failure to provide freedom and prosperity for its people proved the bankruptcy of the Marxist ideology.
For many Marxists, however, the USSR was an example of “State Capitalism”--not “socialism” or “communism”.
Most Marxists explain the Russian revolution’s failure to achieve the democratic goals of socialism by pointing out that socialism is about diverting the goals of production from military competition and profit to the satisfying of human needs. Socialism can therefore hardly be built under conditions of war and violent Western-backed counter-revolution; yet those were precisely the conditions in which the revolutionaries found themselves. A nascent socialist democracy under siege from both within and without is unlikely to emerge from the destruction and threat of war unscathed and undeformed. It is, rather, more likely to become very much like the USSR: a bureaucratic-military-industrial complex dictatorship.
Since socialism is inseparable from democracy (“as air to breathing, as coal to fire, as love to life” as one famous socialist put it), once war conditions prevailed in Russia, and once power had been definitively taken by leaders who were at least once trusted to fight that war, then the very heart of socialism (robust democratic institutions) had been killed, and government of, for, and by the people became government over the people. And it thereby became the anti-thesis of everything the words “union”, “soviet”, “socialist”, and “republic” ever meant.
While desperately unsuccessful at providing the kind of robust democracy of true Marxist aspiration, the USSR was nevertheless very successful indeed in its state capitalist goals of militarizing its economy and achieving superpower status.