Task: Question Generation
Input (English): Passage: After the death of Charles XII, Sweden still refused to make peace with Russia on Peter's terms. Despite a continued Swedish naval presence and strong patrols to protect the coast, small Russian raids took place in 1716 at Öregrund, while in July 1717 a Russian squadron landed troops at Gotland who raided for supplies. To place pressure on Sweden, Russia sent a large fleet to the Swedish east coast in July 1719. There, under protection of the Russian battlefleet, the Russian galley fleet was split into three groups. One group headed for the coast of Uppland, the second to the vicinity of Stockholm, and the last to coast of Södermanland. Together they carried a landing force of nearly 30,000 men. Raiding continued for a month and devastated amongst others the towns of Norrtälje, Södertälje, Nyköping and Norrköping, and almost all the buildings in the archipelago of Stockholm were burned. A smaller Russian force advanced on the Swedish capital but was stopped at the battle of Stäket on 13 August. Swedish and British fleets, now allied with Sweden, sailing from the west coast of Sweden but failed to catch the raiders. After treaty of Frederiksborg in early 1720, Sweden was no longer at war with Denmark, which allowed more forces to be placed against the Russians. This did not prevent Russian galleys from raiding the town of Umeå once again. Later in July 1720 a squadron from the Swedish battlefleet engaged the Russian galley fleet in the battle of Grengam. While the result of the battle is contested, it ended Russian galley raids in 1720. As negotiations for peace did not progress, the Russian galleys were once again sent to raid the Swedish coast in 1721, targeting primarily the Swedish coast between Gävle and Piteå.
Output (English): What event ended Russian galley raids in 1720?