Frederick served in the Russo-Swedish War in Finland in 1788. During the Battle of Hogland, Fredrick wished to send reinforcements by fleet to aid his brother, Duke Charles. This was prevented, and Frederick left the army in protest to Gustav III, whom he referred to as a tyrant. He thereby came to belong to the opposition, and he also became involved in the planned coup by his sister-in-law to depose the monarch; the 1789 Conspiracy. In the autumn of 1789, Hedvig Elisabeth Charlotte prepared to depose Gustav III and place her husband Duke Charles upon the throne. Her ideal was the Swedish Constitution of 1772, which she saw as a good tool for an enlightened aristocracy, and the war and the Union and Security Act had made her a leading part of the opposition. She cooperated with Prince Frederick Adolf and Gustaf Adolf Reuterholm. The plan was to force Charles to act as a symbol of the opposition to the Union and Security Act when the time was right. When the time arrived to make Charles act, however, he refused, which effectively discontinued the coup.