 During the War of 1812, the largely-undefended eastern region of Maine was occupied by British forces, but returned to the United States following failed British offensives on the northern border, mid-Atlantic and south which produced a peace treaty that was to include dedicated land on the Michigan peninsula for Native American peoples.Maine was part of the Commonwealth of Massachusetts until 1820, when it voted to secede from Massachusetts to become a separate state.On March 15, 1820, under the Missouri Compromise, it was admitted to the Union as the 23rd state.