Frederick is a city in, and the county seat, of Frederick County, Maryland. It is part of the Baltimore–Washington Metropolitan Area. Frederick has long been an important crossroads, located at the intersection of a major north–south Indian trail and east–west routes to the Chesapeake Bay, both at Baltimore and what became Washington, D.C. and across the Appalachian mountains to the Ohio River watershed. It is a part of the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area, which is part of a greater Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. The city's population was 65,239 people at the 2010 United States Census, making it the second-largest incorporated city in Maryland, behind Baltimore. Frederick is home to Frederick Municipal Airport (IATA: FDK), which accommodates general aviation, and to the county's largest employer U.S. Army's Fort Detrick bioscience/communications research installation.
Located where Catoctin Mountain (the easternmost ridge of the Blue Ridge mountains) meets the rolling hills of the Piedmont region, the Frederick area became a crossroads even before European explorers and traders arrived. Native American hunters possibly including the Susquehannocks, the Algonquian-speaking Shawnee, or the Seneca or Tuscarora or other members of the Iroquois Confederation) followed the Monocacy River from the Susquehanna River watershed in Pennsylvania to the Potomac River watershed and the lands of the more agrarian and maritime Algonquian peoples, particularly the Lenape of the Delaware valley or the Piscataway and Powhatan of the lower Potomac watershed and Chesapeake Bay. This became known as the Monocacy Trail or even the Great Indian Warpath, with some travelers continuing southward through the "Great Appalachian Valley" (Shenandoah Valley, etc.) to the western Piedmont in North Carolina, or traveling down other watersheds in Virginia toward the Chesapeake Bay, such as those of the Rappahannock, James and York Rivers.
The earliest European settlement was slightly north of Frederick in Monocacy, Maryland. Founded before 1730, when the Indian trail became a wagon road, Monocacy was abandoned before the American Revolutionary War, perhaps due to the river's periodic flooding or hostilities predating the French and Indian War, or simply Frederick's better location with easier access to the Potomac River near its confluence with the Monocacy.