When Elizabeth, his mother's younger sister, became Empress of Russia, she brought Peter from Germany to Russia and proclaimed him her heir presumptive in the autumn of 1742. Previously in 1742, the 14-year-old Peter was proclaimed King of Finland during the Russo-Swedish War (1741–1743), when Russian troops held Finland. This proclamation was based on his succession rights to territories held by his childless great-uncle, the late Charles XII of Sweden, who also had been Grand Duke of Finland. About the same time, in October 1742, he was chosen by the Swedish parliament to become heir presumptive to the Swedish throne. However, the Swedish parliament was unaware of the fact that he had also been proclaimed heir presumptive to the throne of Russia, and when their envoy arrived in Saint Petersburg in November, it was too late. It has been reported that the underage Peter's succession rights to Sweden were renounced on his behalf. Also in November, Peter converted to Eastern Orthodoxy under the name of Pyotr Feodorovich, and was created Grand Duke of Russia. The words "Grandson of Peter the Great" () were made an obligatory part of his official title, underscoring his dynastic claim to the Russian throne, and it was made a criminal offence to omit them.