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The Duchy of Estonia (Swedish: Hertigdömet Estland; Estonian: Eestimaa hertsogkond; German: Herzogtum Estland), also known as Swedish Estonia (Swedish: Svenska Estland), was a dominion of the Swedish Empire from 1561 until 1721 during the time that most or all of Estonia was under Swedish rule. The territory was eventually ceded to Russia in the Treaty of Nystad, following its capitulation during a plague outbreak in the Great Northern War.
The dominion arose during the Livonian War, when the northern parts of present-day Estonia — Reval (Tallinn) and the counties of Harjumaa, Western Virumaa, Raplamaa and Järvamaa — submitted to the Swedish king in 1561, and Läänemaa in 1581. It is also colloquially known as the "good old Swedish times" (Estonian: vana hea Rootsi aeg) by Estonians, but this expression was not used before the following Russian rule, in the beginning of which the situation of Estonian peasantry declined rapidly; to gain the support of the German Baltic nobility, Russia gave them more power over the peasantry.