Complex sentence: The institution of the Pale became more significant following the Second Partition of Poland in 1793, since until then, Russia's Jewish population had been rather limited; the annexation of Polish-Lithuanian territory increased the Jewish population substantially.
Split into simpler sentences: The institution of the Pale became especially important to the Russian authorities following the Second Partition of Poland in 1793. While Russia's Jewish population had, until then, been rather limited, the annexation of Polish-Lithuanian territory increased the Jewish population substantially.