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.
After fortifying Jamestown, colonists built forts at Point Comfort, the mouth of the Hampton River, and at the planned new capital of Henricus. After the start of the Second Anglo-Powhatan War in 1622, the Peninsula was fortified. As settlement spread westward, additional forts were built at the Fall Line.1
Even before the Piedmont was settled, Scotch-Irish and German-speaking immigrants migrated south from Pennsylvania into the Shenandoah Valley starting in the 1730's. Their settlements disrupted the hunting expeditions of the Delaware, Shawnee, and Mingo. Raiding parties of the Haudenosaunee and Cherokee had to avoid the new settlements, but sometimes killed livestock for food while traveling.
Land speculators in Virginia with political connections in the General Assembly and with the governor saw great opportunity in getting royal grants to vast amounts of land west of the Blue Ridge. Using title granted by the British government to hundreds of thousands of acres, the speculators would get richer by surveying and selling parcels in roughly 100-acre chunks to new settlers.
For the large land companies to attract settlers, Native American claims to the land had to be extinguished through private sales or through official government treaties. Not all tribes signed treaties, and not all the leaders within Native American communities endorsed the treaties that were signed, so settlers needed to protect themselves from threats to property and their lives.
After the Haudenosaunee agreed to the Treaty of Lancaster in 1744, Virginia's governors approved more land grants west of the Blue Ridge. The Wood's River Grant was issued in 1745, and the Ohio Company and the Loyal Land Company received grants in 1749.
Some pioneers were unwilling to wait for treaties and land grants. They did not rely upon any legal process to minimize resistance by the indigenous people who lived nearby and hunted throughout the Ohio River watershed. Squatters ignored the issue of ownership, anticipating that their occupation of good bottomlands could be legalized later.
At the start of the French and Indian War, many settlers relied upon their houses to provide protection against the Iroquois, Shawnee, Cherokee or others who simply were passing by. The walls of a fortified house could deter a hunting party that was passing by and not committed to an assault.
If a raiding party was committed to attack, such protection rarely was sufficient for an individual family. A community fort, with multiple families inside, had far more firepower and capacity to repel an attack. Building earthworks and palisades around communal forts and collecting more fighters inside did offer more protection that individual homes, but the forts of the 1750's were still vulnerable to raiding parties - even though they did not have artillery that could knock down walls.
Fort Vause (also known as Fort Vass) was constructed in 1753 by settlers in the New River near modern-day Radford and a few in the headwaters of the Tennessee River. After a Shawnee raid in 1755, many settlers along the New River fled to it for protection. Among them were Mary Draper Ingles and William Ingles.
Mary Draper Ingles already had escaped from captivity in Kentucky and managed to walk home. After a few days at Fort Vause, she convinced her husband (apparently based on a premonition) to leave Fort Vause and move east of the Blue Ridge to a safer location in Bedford County. They left early on June 7, 1756.
Later that day, a Shawnee attack captured Fort Vause. Everyone taking shelter within its walls were killed or captured. Two of William Ingles' brothers were among those who were killed. The raiding party included a French officer, Marie Francois Picoté, Sieur de Belestre II.2

Vause's Fort was near the modern community of Shawsville in Motgomery County
Source: ESRI, ArcGIS Online
After Braddock's defeat in 1755 near Fort Duquesne, all the British forces who had arrived with Braddock retreated east to Philadelphia. The Virginia colony had to rely upon its own resources to deal with a Native American war, with weapons and some officers supplied by the French based at Fort Duquesne.
Recognizing that the British Regulars had abandoned Virginia and Braddock's army was not going to fight again that year, the colony raised an army and built a series of forts on the western edge of settlement.
Gov. Dinwiddie appointed George Washington a Commander-in-Chief of all the Virginian forces on August 14, 1755, after it became clear that:3

George Washington, dressed in his Virginia militia uniform
Source: Library of Congress, George Washington Papers
The General Assembly in Williamsburg recycled the defensive strategy that had been adopted several times in the 1600's. As in the 1600's, the number of available militia soldiers was so small and the extent of territory to be guarded was large. Once again, the colony built forts on the western edge of settlement and relied upon small parties of rangers based at the forts to intercept raiding parties.
The failure of colonial leadership to protect the colonists in 1676 had led to Bacon's Rebellion. Angry settlers organized an unauthorized army and attacked Native American communities. Expressing their frustration with colonial leadership, the rebels burned the colonial capital of Jamestown and Governor William Berkeley was forced to flee to the Eastern Shore.
The key difference in the 1750's would be the commitment of enough British troops, shipped across the Atlantic Ocean, to take the offensive and destroy the towns and fortifications of the enemy. Building defensive forts west of the Blue Ridge was just the initial reaction after Braddock's army was led east to the Delaware River.
Washington recognized from the start that that forts located 20 miles from each other offered little protection to most settlers. They might flee to their individual homes, but would be captured/killed there by raiding parties before reinforcements from a fort could arrive. A line of widely-scattered forts would not be an effective barrier to Shawnee raids.
Washington shared his opinion with Governor Dinwiddie in early 1756:4
On November 9, 1756, Washington sent Governor Dinwiddie his plan for constructing a chain of forts roughly 15 miles apart. Like the individual houses fortified by settlers, the colonial forts were to be used for defensive purposes.
Washington did not suggest placing detachments of the Virginia Regiment at Fort Cumberland in Maryland, which had supported General Braddock's ill-fated march to capture Fort Duquesne in 1755. He viewed a fort at Cumberland to be suitable for just offensive operations. The forts in Virginia were designed to resist attack, with the assumption based on experience that Native American raiders would rarely besiege a location for more than several days.
Governor Dinwiddie and the General Assembly were requested to pay for a total of 2,000 militia. They would be recruited to serve with pay as "Provincials" in the Virginia Regiment, since British Regulars were not available.
Fort Vause, which was being rebuilt by Captain Peter Hogg, needed 150 men. It was "in a much exposed gap, subject to the inroads of the southern Indians," and had to provide protection for both Bedford and Halifax counties east of the Blue Ridge.

in the 1750's, settlers were too scattered to justify building a colonial fort between Fort Vause/Vass on the Roanoke River and Fort Loudoun on the Little Tennessee River
Source: Library of Congress, A map of the most inhabited part of Virginia containing the whole province of Maryland with part of Pensilvania, New Jersey and North Carolina. (by Joshua Fry and Peter Jefferson, 1755)

in 1756, George Washington recommended staffing 22 forts in the backcountry with 2,000 militia
Source: Founders Online, Enclosure: Proposal for Frontier Forts, 9 November 1756 (by George Washington)
Washington viewed the most likely sites of attack to be near the Potomac River. In response, he proposed placing 500 men at Cocke's Fort on Patterson Creek near modern Heathsville, West Virginia. He recommended stationing 250 men at Dickinson's Fort, later known as Fort Young, on the Jackson River in modern Covington.


in 1756, Washington proposed placing 500 militia at Cocke's Fort and 250 men at Dickenson's Fort
Source: ESRI, ArcGIS Online; ArcGIS online
Fort Loudoun in Winchester served as Washington's headquarters. It as the largest facility, able to house 450 men and host 24 cannon.5

the largest in the 1756 defensive line of forts was Fort Loudoun in Winchester
Source: David Rumsey Map Collection, Fort Loudoun at Winchester, Virginia, drawn by George Washington about 1776
During George Washington's tour of the backcountry defenses in 1756, he was frustrated at the slow pace of construction at what he called "Fort Vass" as well as the unexpectedly high cost.
According to local lore, Shawnee raiders in the area were aware that Colonel Washington was coming to inspect the fort. Supposedly a party planned an ambush, but the leader stepped away for a moment and commanded the other Native Americans not to fire until he returned. In the interval, Washington rode safely into the fort.
Fort Cumberland in Maryland was attacked in 1757 in another raid led by Marie Francois Picoté, Sieur de Belestre II. The French officer was captured soon afterwards near Fort Duquesne by a scouting party of the Virginia Regiment that included 15 Cherokee.6
The Virginia forces included rangers as well as militia stationed at the forts. After gaining experience in 1753-1755, George Washington recognized the value of having Native American allies. In a letter to Governor Dinwiddie sent from Winchester in September 1756, Washington highlighted the difficulty of recruiting men to serve in the Virginia Regiment - and also the need to recruit Native Americans.
Since the tribes living in the Ohio River Valley were working with the French, Washington sought to recruit from the Carolinas:7
Recruiting the Cherokees, Catawbas, Nottoways, Tuscaroras, Meherrins, Saponis, and Creeks to assist the Virginians was key to gathering intelligence and tracking Shawnee, Delaware and Mingo raiding parties. The Cherokee quickly concluded that the colonists did not add value to their expeditions. Christopher Gist was instructed in 1757 to keep the Virginia Regiment officers from interfering in the military tactics of the Native American allies."8
As anticipated by George Washington, the forts near the Potomac River were used for offensive operations. Native Americans coming from the Carolinas would be supplied at Winchester, then sent to Pearsall's Fort (near modern Romney, West Virginia) or Fort Cumberland (modern Cumberland, Maryland). In 1757-1758, warriors and Virginians based at those forts attacked towns in the Ohio River watershed. The threat of the Cherokee helped, as Washington intended, to "keep their Indians at home."9
There were hundreds of small farms in the Holston River and Clinch River watersheds. Many assumed they were located in Virginia, but the boundary line with the North Carolina colony was undefined. The primary issue for those settlers was getting the Cherokee to accept the settlements rather than to attack them.
Fort Loudoun was constructed in 1756-67 by South Carolina militia, who wanted to maintain the fur trade of the Overhill Towns of the Cherokee with Charles Town. The peace between colonists and the Cherokee ended in 1760. South Carolinians massacred Cherokee chiefs being kept as hostages at Fort Prince George. (The site of Fort Loudoun is now underneath Tellico Lake, while Fort Prince George is beneath Lake Keowee.)
The Cherokee then besieged and captured Fort Loudoun.

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)