The collapse of the Soviet Union in 1991 reversed a process that had been under way since the Russian Empire's emergence in the 17th century. It was ultimately to incorporate four general elements: Eastern Europe, Central Asia, the Caucasus and Siberia. The St. Petersburg-Moscow axis was its core, and Russia, Belorussia and Ukraine were its center of gravity. The borders were always dynamic, mostly expanding but periodically contracting as the international situation warranted. At its farthest extent, from 1945 to 1989, it reached central Germany, dominating the lands it seized in World War II. The Russian Empire was never at peace. As with many empires, there were always parts of it putting up (sometimes violent) resistance and parts that bordering powers coveted -- as well as parts of other nations that Russia coveted.
The Russian Empire subverted the assumption that political and military power requires a strong economy: It was never prosperous, but it was frequently powerful. The Russians defeated Napoleon and Hitler and confronted the far wealthier Americans for more than four decades in the Cold War, in spite of having a less developed or less advanced economy. Its economic weakness certainly did undermine its military power at times, but to understand Russia, it is important to begin by understanding that the relationship between military and economic power is not a simple one.
Economy and Security
There are many reasons for Russia's economic dysfunction, but the first explanation, if not the full explanation, is geography and transportation. The Russians and Ukrainians have some of the finest farmland in the world, comparable to that of the American Midwest. The difference is transportation, the ability to move the harvest to the rest of the empire and its far-away population centers. Where the United States has the Mississippi-Missouri-Ohio river system that integrates the area between the Rockies and the Appalachians, Russia's rivers do not provide an integrated highway to Russia, and given distances and lack of alternative modes of transport, Russian railways were never able to sustain consistent, bulk agricultural transport.
This is not to say that there wasn't integration in the empire's economy and that this didn't serve as a factor binding it together. It is to say that the lack of economic integration, and weakness in agricultural transport in particular, dramatically limited prosperity in the Russian Empire and the Soviet Union. At the same time, the relative underdevelopment of the empire and union made it impossible for them to successfully compete with Western Europe. Therefore, there was an economic motivation within the constituent parts of the empire and the union to integrate with each other. There could be synergies on a lower level of development among these nations.
Economics was one factor that bound the Russian Empire and Soviet Union together. Another was the military and security apparatus. The Russian security apparatus in particular played a significant role in holding first the empire and then the union together; in many ways, it was the most modern and efficient institution they had. Whatever temptations the constituent republics might have had to leave the empire or union, these were systematically repressed by internal security forces detecting and destroying opposition to the center. It could be put this way: The army created the empire. Its alignment of economic interests was the weak force holding it together, and the security apparatus was the strong force. If the empire and union were to survive, they would need economic relations ordered in such a way that some regions were put at a disadvantage, others at an advantage. That could happen only if the state were powerful enough to impose this reality. Since the state itself was limited in most dimensions, the security apparatus substituted for it. When the security apparatus failed, as it did at the end of World War I or in 1989-1991, the regime could not survive. When it did succeed, it held it all together.