When the Russo-Swedish War broke out in 1741, the government of Anna Leopoldovna appointed him Commander-in-Chief as the most experienced among Russian generals. Lacy quickly struck against Finland and won his last brilliant victory at Lappeenranta . The following year he rallied his forces and proceeded to capture Hamina, Porvoo and Hämeenlinna, by August encircling more than 17,000 Swedes near Helsinki and effectively bringing the hostilities to an end. The war over, Lacy withdrew to Riga and resumed the command of the Russian forces stationed in Livland. He administered what is now Northern Latvia and Southern Estonia until his death on 30 April 1751 in Riga. His son Franz Moritz von Lacy had entered the Austrian service in 1743 and became one of the most successful imperial commanders of the 18th century.. How many years did the Russo-Swedish War last?