Complex sentence: Sweden, Russia's only ally, did not send supporting troops, but the alliance made it possible to withdraw the 45,000-man Russian corps Steinheil from Finland and use it in the later battles (20,000 men were sent to Riga).
Split into simpler sentences: Sweden, Russia's only ally, did not send supporting troops. But the alliance made it possible to withdraw the 45,000-man Russian corps Steinheil from Finland and use it in the later battles (20,000 men were sent to Riga).