Irritable and suspicious like his brother he also came to the conclusion that his services had not been adequately appreciated, and the flattering way in which he was welcomed by the Russian court during a visit to St. Petersburg in 1779 still further incensed him against the purely imaginary ingratitude of his own sovereign. For the next two years he was in the French service, returning to Finland in 1781. It was now that he first conceived the plan of separating the grand duchy from Sweden and erecting it into an independent state under the protection of Russia. During the Riksdag of 1786 he openly opposed Gustav III of Sweden, at the same time engaging in a secret and treasonable correspondence with the Russian ministers with the view of inducing them to assist the Finns by force of arms.
In the following year, at the invitation of Catherine II of Russia, he formally entered the Russian service. When the Russo-Swedish War of 1788-1790 began, Sprengtporten received the command of a Russian army corps directed against Finland. He took no direct part in the Anjala conspiracy but urged Catherine to support it more energetically. His own negotiations with his fellow countrymen, especially after Gustav III of Sweden had brought the Finlanders back to their allegiance, failed utterly. Nor was he able to serve Russia very effectively in the field for he was seriously wounded at the battle of Parosalmi 1790. At the end of the war, indeed, his position was somewhat precarious, as the High Court of Finland condemned him as a traitor, while Catherine regarded him as an incompetent impostor who could not perform his promises. For the next five years, therefore 1793-1798, he thought it expedient to quit Russia and live at Teplice in Bohemia. He was re-employed by the emperor Paul of Russia who, in 1800, sent him to negotiate with Napoleon concerning the Maltese Order and the interchange of prisoners. After Paul’s death Sprengtporten was again in disgrace for seven years, but was consulted in 1808 on the eve of the outbreak of hostilities with France. On December 1, 1808 he was appointed the first Russian Governor-General of Finland with the title of count, but was so unpopular that he had to resign his post the following year. The last ten years of his life were lived in retirement.
See also: List of Swedish politicians