As commander of the Free Corps he defeated a somewhat larger Prussian force at the 1761 Battle of Neuensund, and was one of the few officers to emerge with any credit from what was generally an unsuccessful war for Sweden.

[2][3] Sprengtporten's status as a war hero, and his prior familiarity with the Finnish fortifications, led to his being selected in 1766 to compose a report on the state of Finland's border defences, and his subsequent lobbying of the government to implement his recommendations drew him into politics for the first time.

The Hats enjoyed strong support among the aristocracy and army officers, and Sprengtporten was sympathetic to their views and sometimes seen as one of their leaders, but he was frustrated by the indecision, capriciousness and short-termism of parliamentary government and became convinced that the constitution had to be changed to create a stronger executive.

They planned that Sprengtporten would lead a mutiny among the large garrison at Sveaborg Fortress, in his native Finland, and then ship these troops over to Stockholm to seize the capital and overthrow the government.

[9][10] King Gustav handsomely rewarded Sprengtporten for his support, most notably by promoting him to the rank of lieutenant-general and giving him command of the Life Guards regiment in which he had begun his military career thirty years earlier.

However, Sprengtporten was embarrassed by his failure to arrive in time to assist with the coup in Stockholm, and he became deeply insecure about the matter, and also jealous of the king for taking the role of heroic revolutionary leader which he had hoped to fulfill himself.

He again threatened to resign over the winter of 1773–4, after the king only partially accepted his recommendations concerning the defence of Finland against a possible Russian invasion, and finally went through with the threat in the spring of 1774 after failing to get his way over a trivial issue relating to a brawl between soldiers from different regiments.