As Governor-General he commanded Swedish forces in 1675/76 against the allied German armies during the Bremen-Verden Campaign, but eventually had to surrender. He was given the command of the Royal Swedish Navy in 1677 and was made a member of the Privy Council of Sweden ("riksråd"). That same year, he was defeated by Denmark in the Battle of Køge Bay during the Scanian War. He was appointed to the head of the College of Amiralities in 1677. In 1678, he was appointed as the Commander-in-Chief of Livonia which was a dominion of the Swedish Empire from 1629 until 1721. In 1680, Horn left Livonia and returned to help rebuild Stade which had been besieged in 1676 during the Swedish-Brandenburg War.