Gustav IV Adolf had refused Russian demands that Sweden join Napoleon's Continental System and break with Britain. On 21 February 1808, twenty-four thousand Russian troops crossed the frontier without a declaration of war. By March they held Helsinki, Turku and Vaasa. The worst blow to the coast fell before a shot was fired at sea: the Swedish archipelago fleet had been laid up for the winter at Åbo, and the Swedes burned their own ships rather than let them be captured. What was left had to fight an entire summer along one of the most complicated coastlines in Europe, against a Russian coastal force that eventually numbered around a hundred vessels. The fighting moved island by island through the summer of 1808, at Rimito Kramp at the end of June, at Sandöström on 2 August, and finally in the sounds off Kustavi.

The gun sloop was a specialist's answer to a specialist's problem. Between fifteen and nineteen metres long, three and a half to four metres across the beam, drawing rather less than a metre of water, it could go almost anywhere a rowboat could go. Ten to twelve pairs of oars drove it, two men to an oar, with a pair of collapsible sloop-rigged masts for open stretches. Onto that slim open hull the shipwrights bolted a twelve- or twenty-four-pounder gun at the bow and another at the stern, plus a three-pounder swivel on each side. Fifty to sixty-four men crewed one. Nothing about the design offered any protection at all. The men rowing sat in the open, at eye level with the water, inside a wooden shell narrower than a modern living room, and there was nowhere on board to be safe.

Lieutenant Colonel Johan Ludvig Brant had been given the job of clearing Russian coastal forces working the waters around Nystad, the town the Finns call Uusikaupunki. When he learned that roughly twenty Russian gun sloops and yawls had left Nystad heading for Turku, he set off after them with a force nearly twice their size: thirty-five sloops, with Major Sölfverarm and Captain Carl Ulrik af Klercker under him. The Russians saw the Swedes coming and formed up rather than run. Brant tried to work a detachment around the island that dominated the channel and take them from behind, but Selivanov read the move and blocked it. So the Swedes came in straight and close instead. With the wind favouring them and their numbers telling, they still could not break the Russian line; Selivanov gave ground slowly, keeping his squadron together, until his ammunition ran out at seven in the evening.

Nine Russian sloops went down. Two Swedish ones did. Read the butcher's bill instead of the ship count and the day looks different: the Swedes counted 225 men killed or wounded, the Russians 114, and the Russian figure is known to be incomplete. The side that won lost roughly twice as many men as the side that retreated. That is what close-range gunnery does to open boats packed with rowers. One reason the Russian toll was not far worse is that Brant chose not to board; hand-to-hand fighting across gunwales would have finished the crippled sloops but at a cost he was unwilling to pay. Almost none of these men are named in the record. They were Finnish, Swedish and Russian conscripts and seamen doing very hard physical labour, in the open, while iron came through the planking beside them.

Grönvikssund did what a tactical victory can sometimes do: it bought time. The Russians shelved their plan to carry the war across to Åland, and their coastal forces spent the following weeks escorting supply convoys instead of forming a landing force. That was the whole of it. By November 1808 Russia controlled all of Finland. In March 1809 a coup deposed Gustav IV Adolf in Stockholm, and on 17 September 1809 the Treaty of Fredrikshamn transferred Finland and Åland to the Russian Empire, which reconstituted them as an autonomous Grand Duchy. The sounds where Brant's sloops chased Selivanov's into the dark became internal Russian water overnight. Today they are quiet summer boating channels, and the only monuments are the ones nobody built.