Passage: Åland is a collection of around 6,500 islands in the Baltic Sea, midway between Sweden and Finland. The islands are almost exclusively Swedish-speaking, but in 1809, the Åland Islands, along with Finland, were taken by Imperial Russia. In December 1917, during the turmoil of the Russian October Revolution, Finland declared its independence, but most of the Ålanders wished to rejoin Sweden. The Finnish government considered the islands to be a part of their new nation, as the Russians had included Åland in the Grand Duchy of Finland, formed in 1809. By 1920, the dispute had escalated to the point that there was danger of war. The British government referred the problem to the League's Council, but Finland would not let the League intervene, as they considered it an internal matter. The League created a small panel to decide if it should investigate the matter and, with an affirmative response, a neutral commission was created. In June 1921, the League announced its decision: the islands were to remain a part of Finland, but with guaranteed protection of the islanders, including demilitarisation. With Sweden's reluctant agreement, this became the first European international agreement concluded directly through the League.

Question: When did Alanders rejoin Sweden?
December 1917