On March 15, 1820, Maine was admitted as the 23rd state of the Union. From the article:
"Maine enters the Union
As part of the Missouri Compromise between the North and the South, Maine is admitted into the Union as the 23rd state. Administered as a province of Massachusetts since 1647, the entrance of Maine as a free state was agreed to by Southern senators in exchange for the entrance of Missouri as a slave state.
In 1604, French explorer Samuel de Champlain visited the coast of Maine and claimed it as part of the French province of Acadia. However, French attempts to settle Maine were thwarted when British forces under Sir Samuel Argall destroyed a colony on Mount Desert Island in 1613. Sir Ferdinando Gorges, a leading figure in the Plymouth Company, initiated British settlement in Maine after receiving a grant and royal charter, and upon Gorges’ death in 1647 the Massachusetts Bay Colony claimed jurisdiction. Gorges’ heirs disputed this claim until 1677, when Massachusetts agreed to purchase Gorges’ original proprietary rights.
As part of Massachusetts, Maine developed early fishing, lumbering and shipbuilding industries and in 1820 was granted statehood. In the 19th century, the promise of jobs in the timber industry lured many French Canadians to Maine from the Canadian province of Quebec, which borders the state to the west. With 90 percent of Maine still covered by forests, Maine is known as the “Pine Tree State” and is the most sparsely populated state east of the Mississippi River."