The northern British colonies had their own name for Louisbourg: the "American Dunkirk." French privateers operating from its harbor preyed on New England shipping and fishing vessels, while the fortress itself protected the entrance to Canada and the rich Grand Banks fisheries. Intermittent warfare between the French, their Wabanaki Confederacy allies, and the New England colonies had created decades of resentment along the northeastern frontier. When war broke out again in 1744 as part of the War of the Austrian Succession -- known in the colonies as King George's War -- the French launched raids that reached Canso and threatened Annapolis Royal. Massachusetts Governor William Shirley saw an opportunity. He proposed an expedition against Louisbourg to the General Court, which approved it by a margin of just one vote. Connecticut, New Hampshire, and Rhode Island contributed troops and ships, assembling roughly 4,200 provincial soldiers supported by a British naval squadron under Commodore Peter Warren.

What the colonial attackers lacked in military training, the French garrison matched in dysfunction. The fortress troops were poorly paid and supplied, and a mutiny had broken out just months before the siege. The garrison's inexperienced leaders distrusted their own soldiers. Meanwhile, the fortress itself had a design flaw that French engineers had never adequately addressed: while its seaward defenses were formidable, a series of low rises behind the walls provided natural positions for siege batteries. The colonial force landed at Gabarus Bay on April 30, 1745, about three miles from the fortress, wading through heavy surf and exchanging fire with French defenders. William Pepperrell, commanding his first military operation, organized his untrained troops with a merchant's pragmatism rather than a general's doctrine. What followed was less a textbook siege than a sustained exercise in colonial stubbornness.

The colonists' first major stroke of luck came almost immediately. Upon landing, the provincial forces advanced on the Royal Battery, a massive fortification across the harbor that mounted 40 cannons protecting the harbor entrance. The French, panicked by the speed of the colonial advance, abandoned the battery with much of its armament still operational. The New Englanders occupied it and turned the guns on the fortress -- a gift of firepower they could never have brought across the Atlantic themselves. A French and Indian counterattack the next day was repulsed. Over the following weeks, the colonists hauled cannons through bogs and over rough terrain to establish additional batteries, including positions on the hills behind the fortress that exploited Louisbourg's known blind spot. The siege settled into a grinding artillery duel punctuated by failed assaults, including a costly nighttime attack on the Island Battery that was repulsed with significant colonial casualties.

By mid-June, weeks of bombardment had battered Louisbourg's walls and demoralized the garrison. The Vigilant, a French warship carrying supplies and reinforcements, was intercepted by Warren's naval squadron before it could reach the harbor, eliminating the garrison's last hope. On June 28, 1745, the French commander Louis Du Pont Duchambon surrendered the fortress. The New England troops occupied Louisbourg and held it through a miserable winter during which disease killed more men than the siege had. Then came the blow that colonists remembered for a generation: in the Treaty of Aix-la-Chapelle ending the war in 1748, Britain traded Louisbourg back to France in exchange for Madras in India and border adjustments in the Low Countries. The colonists who had fought and died for the fortress watched European diplomats give it away for territories they had never seen. The resentment this bred toward imperial decision-making would echo three decades later, when the same colonies began questioning why they owed loyalty to a crown that treated their sacrifices as bargaining chips.