George Washington, dressed in his Virginia militia uniform
Source: Library of Congress, George Washington Papers
Since English colonists arrived in Virginia, they built forts to protect themselves from attack. The need for such protection was demonstrated at Jamestown within the first month of settlement. 1 In Tidewater,
After Braddock's defeat in 1755 near Fort Duquesne, all British forces who had arrived with Braddock retreated into Pennsylvania. Once again, the Virginia colony had to rely upon its own resources to deal with a Native American war.
The General Assembly in Williamsburg recycled the defensive strategy adopted in 1676 - build a series of forts to protect farms and settlements. That strategy had been an absolute failure, back when the frontier was further east at the boundary of the Piedmont and Coastal Plain. The failure of colonial leadership to protect the colonists led to Bacon's Rebellion, when the colonial capital of Jamestown was burned and Gov. William Berkeley was forced to flee to the Eastern Shore.
Gov. Dinwiddie appointed George Washington as head of the Virginia militia. Washington recognized that forts located 20 miles from each other offered little protection to most settlers, who would be captured/killed before they could flee to safety. A line of widely-scattered forts would not be an effective barrier to Shawnee raids, so Washington wrote Governor Dinwiddie in early 1756:2
George Washington, dressed in his Virginia militia uniform
Source: Library of Congress, George Washington Papers
after the 1768 Treaty of Fort Stanwix, Fort Pitt was on the edge of authorized colonial settlement
Source: Library of Congress, Cantonment of His Majesty's forces in N. America according to the disposition now made & to be compleated as soon as practicable taken from the general distribution dated at New York 29th. March 1766
General Gage closed Fort Chartres in 1772 in order to reduce the costs of maintaining the British army in North America
Source: William L. Clements Library, University of Michigan, Map of the frontiers of the northern colonies with the boundary line established between them and the Indians at the treaty held by S. Will. Johnson at Ft. Stanwix in Novr. 1768 (by Guy Johnson)
western forts were close to Native American towns, and centers of trade and interaction between colonial and Native American societies
Source: Library of Congress, Map of the Ohio River from Fort Pitt (by John Montrésor, 1776)
Fort Pitt disappeared quickly after the American Revolution, as the city of Pittsburgh developed
Source: New York Public Library, Map of Pittsburgh and its environs (by Lewis Kenyon, 1830)