Maine became the 23rd state of the United States on March 15, 1820. It was part of the Missouri Compromise, which aimed to maintain a balance between free and slave states. Before statehood, Maine was a part of Massachusetts and had a growing population that sought independence. The process of achieving statehood involved significant political negotiations. Maine's admission as a free state allowed Missouri to enter as a slave state, helping to ease tensions between the North and South. This compromise was crucial in shaping the future of the United States during a time of increasing sectional conflict.