Complex sentence: Although all republics were equal under union law, for its entire existence the Soviet Union was dominated by the Russian republic--by far the largest, in both population and geography, as well as the strongest and most developed economically due to its vast natural resources.
Split into simpler sentences: Although all republics were equal under union law, for its entire existence the Soviet Union was dominated by the Russian republic. The RSFSR was by far the largest republic, in both population and geography, as well as the strongest and most developed economically due to its vast natural resources.