The Pine Tree State became the 23rd state in the Union on March 15, 1820. After much negotiation and discussion, it was agreed that Maine, which had been part of Massachusetts, would be admitted to the Union as a free state and Missouri was admitted as a slave state. All land west of Missouri's southern border was made free. Celebrate your state's history by displaying state symbols, animals, and state flags. Find your supplies at Flagandbanner.com today.