In his twelfth year Sprengtporten chose the profession of arms, and served his country with distinction. The few and miserable triumphs of Sweden during the Seven Years' War were due almost entirely to young Sprengtporten, and he emerged from it with a lieutenant-colonelcy, a pension and the reputation of being the smartest officer in the service.
Sprengtporten, above all things a man of action, had too hearty a contempt for "Hats" and "Caps" to belong to either. He regarded the monstrous system of misrule for which they were primarily responsible with indignation, made no secret of his sentiments, and soon gathered round him a band of young officers of strong royalist proclivities, whom he formed into a club, the so-called Svenska Botten (Sweden's groundwork). The club was suppressed by the dominant "Caps, " who also sought to ruin Sprengtporten financially by inciting his tenants in Finland to bring actions against him for alleged extortion, not in the ordinary courts but in the riksdag itself, where Sprengtporten's political adversaries would be his judges. The enraged Finnish colonel thereupon approached Gustavus III with the project of a revolution against their common enemies, the "Caps. " It was to begin in Finland where Sprengtporten's regiment, the Nyland dragoons, was stationed. He undertook to seize the impregnable fortress of Sveaborg by a coup de main. The submission of the whole grand duchy would be the natural consequence of such a success, and, Finland once secured, Sprengtporten proposed at the head of his Finns to embark for Sweden, meet the king and his friends near Stockholm, and surprise the capital by a night attack. This plan, subsequently enlarged by a suggestion of a fellow plotter, Johan Christopher Toll, was warmly approved of by the king.
On 22 July 1772 Sprengtporten left Stockholm. On 9 August he reached Helsingfors. On 16 August he persuaded to submit to him fortress of Sveaborg, soon followed by that of Helsingfors. A week later all Finland was under the colonel's control. By 23 August Sprengtporten was ready to re-embark for Stockholm with 780 men, but contrary winds kept him back, and in the meantime Gustav III himself had carried out his revolution unaided. On his return to Sweden, however, Sprengtporten was received with the greatest distinction and made a lieutenant-general and colonel of the guards. He was also appointed the president of a commission for strengthening the defences of Finland. But Sprengtporten was still dissatisfied. He could never forgive Gustav for having forestalled the revolution, and his morbidly irritable and suspicious temper saw slights and insults in the most innocent conjunctures. His first quarrel with Gustav happened in 1774 when he refused to accept the post of commander-in-chief in Finland on the eve of threatened war with Imperial Russia. The king good-naturedly overlooked his outrageous insolence on this occasion, but the inevitable rupture was only postponed. A most trumpery affair brought matters to a head. Sprengtporten had insulted the guards by giving precedence over them at a court-martial to some officers of his own dragoons. The guards complained to the king, who, after consulting with the senate, mildly remonstrated with Sprengtporten by letter.
Sprengtporten thereupon tendered his resignation as colonel of the guard, and at a personal interview with Gustav was so violent and insolent that anything like agreement between them became impossible. Sprengtporten was haunted by the fixed idea that the "jeunesse dorée" of the court was in league with his old enemies to traduce and supplant him, and not all the forbearance of the king could open his eyes. He received a pension on his retirement and was allowed the extraordinary privilege of a guard of honour as long as he lived. Nevertheless, to the end of his career, he continued to harass and annoy his long-suffering benefactor with fresh impertinences.