During the colonial era, families of free people of color were formed most often by unions of white women and African men. Many of the free black families migrated to Delaware, where land was cheaper. As the flow of indentured laborers to the colony decreased with improving economic conditions in England, planters in Maryland imported thousands more slaves and racial caste lines hardened. The economy's growth and prosperity was based on slave labor, devoted first to the production of tobacco as the commodity crop. Maryland was one of the thirteen colonies that revolted against British rule in the American Revolution. Near the end of the American Revolutionary War (1775–1783), on February 2, 1781, Maryland became the last and 13th state to approve the ratification of the Articles of Confederation and Perpetual Union, first proposed in 1776 and adopted by the Second Continental Congress in 1778, which brought into being the United States as a united, sovereign and national state. It also became the seventh state admitted to the Union after ratifying the new federal Constitution in 1788. In December 1790, Maryland donated land selected by first President George Washington to the federal government for the creation of the new national capital of Washington, D.C. The land was provided along the north shore of the Potomac River from Montgomery and Prince George's counties, as well as from Fairfax County and Alexandria on the south shore of the Potomac in Virginia; however, the land donated by the Commonwealth of Virginia was later returned to that state by the District of Columbia retrocession in 1846. Influenced by a changing economy, revolutionary ideals, and preaching by ministers, numerous planters in Maryland freed their slaves in the 20 years after the Revolutionary War. Across the Upper South the free black population increased from less than 1% before the war to 14% by 1810. Abolitionist Harriet Tubman was born a slave during this time in Dorchester County, Maryland. During the War of 1812, the British military attempted to capture Baltimore, which was protected by Fort McHenry. During this bombardment the song "Star Spangled Banner" was written by Francis Scott Key; it was later adopted as the national anthem. The National Road (U.S. Hwy 40 today) was authorized in 1817 and ran from Baltimore to St. Louis—the first federal highway. The Baltimore and Ohio Railroad (B&O) was the first chartered railroad in the United States. It opened its first section of track for regular operation in 1830 between Baltimore and Ellicott City, and in 1852 it became the first rail line to reach the Ohio River from the eastern seaboard.