America was built by territorial conquest, so we thought like conquerors and assumed everyone else did. But Russia's history is one of constantly being invaded or conquered, so the Soviets' mentality was one of protecting themselves from the rivals or enemies who surrounded them on all sides.
Russia (under the Tsar) had plenty of territorial conquest in its history; the continuous eastward expansion across northern Asia to the Pacific took place over a similar period of time as the overseas empire-building the western European colonial powers were involving themselves in. Indeed, throughout the nineteenth century, both the Chinese and British (in India) were all too aware of how uncomfortably close the Russian Empire's expanding frontiers were to their own areas of influence. (In the case of the Chinese, some of the Russians' new territories had been seized from them directly; the area around the modern city of Vladivostok was handed over as recently as 1860.)
The main difference is that, with the exception of Alaska, none of those new acquisitions were "overseas", the way, say, Australia was for the British, or Brazil was for the Portuguese. While the process of decolonisation was by no means clean and bloodless for the western powers, there was still that sense of distance and apart-ness which, in the long run, made decoupling from these colonies that much easier. (Arguably, the same was true for the Russians; their attitude about selling Alaska to the United States was very different to how they felt about their holdings in Eurasia.)
Funnily enough, the very idea of "where is Russia" as opposed to "where is the Russian Empire" only really came into focus under the Soviet lens. When the borders were drawn up for the RSFSR, it did more than establish a territorial sub-division of the Soviet Union; it marked what would be seen as part of Russia (and thus to be inherited by the modern Russian Federation) and what was part of the wider USSR (which would be given over to the other CIS countries). Although, given that much of
that territory was
still on lands conquered by the old Tsardom, it was still a less than tidy solution; not least since places like Chechnya were placed within the boundaries marked for the RSFSR, not beyond them.
In terms of Soviet (as opposed to Tsarist) ambitions, I'd argue that it was only after Stalin died that the USSR gave up any particular ambitions of further expansion, at least directly. Indirectly, they were still up for supporting newly-emerging friends in other parts of the world, but the degree to which they were invested in this was that bit different.
(You could argue something similar about the PRC's shift in policy in the wake of Mao's death. When Mao was in power, the change in attitude towards the USSR shifted considerably after Stalin died, worsening relations between the two powers significantly. It was only after Mao's death, when the type of authority in Beijing changed itself, when tensions started to cool down along the Sino-Soviet border again.)
Oh, regarding the
Balance of Terror events; as a point of comparison, the events portrayed there did spark a brief war with the Federation over in the Star Fleet Universe. There, the war was ended by a new treaty, which re-established the Neutral Zone between the two empires.
In the SFU, the Romulans at that time were still saddled with "non-tactical warp"; ships that could use their impulse engines to travel across interstellar space, but were only able to enter conbat at sublight speeds. The Federation had long advanced to use "tactical warp"; with antimatter-based engines which allowed them to fight at low warp speeds, and to move far more quickly at a strategic level. (Tactical warp also allows for the deployment of other modern systems, like phasers, photon torpedoes, transporters, and so forth.)
The failure in that war showed in stark terms how far behind the Romulans truly were, and provided one of the key motivators between their later treaty with the Klingons. (While the Romulans were, slowly, making some kind of progress with their own tactical warp engines, the Klingons pre-empted this by essentially jump-starting the Romulan technology base themselves.)
Actually, this month's
Captain's Log is set to include a story set during that war, looking at one of the other Federation ships to be caught in the crossfire.
Also, the SFU Romulans are pretty clear in their wider intent; their official aim is to go out and claim the entire galaxy for themselves. (Naturally, the Klingons had other ideas, but helping the Romulans served their ends at the time of the treaty.) The wider scope of Romulan ambitions in the Franchise are less clearly defined, though the "Principle of Unlimited Expansion" mentioned by Admiral Valdore in
ENT does ring a familiar bell...
)
