Fort Beausejour rose from the low marshlands of the Isthmus of Chignecto, the narrow strip of land connecting Nova Scotia to the mainland. The 1713 Treaty of Utrecht had ceded mainland Nova Scotia to Britain but left the boundaries deliberately vague. The de facto border settled along the Missiguash River, where the prosperous Acadian settlement of Beaubassin straddled the divide. In 1750, when British Major Charles Lawrence arrived with troops to assert authority, French commander Louis de La Corne made a ruthless choice: he evacuated and burned Beaubassin entirely rather than let it provision the British. Over the ashes of that village, two forts rose facing each other across the marshes. The French built Beausejour on the north bank. The British built Fort Lawrence on the south. For five years, they watched each other across a river barely a musket shot wide.

Beausejour was a pentagon-shaped earthwork fort with bastions at each corner, completed between 1751 and 1752. It was built to protect the vital land corridor between Louisbourg and Quebec, the only overland route connecting France's Atlantic fortress with the heart of New France. The fort was more than a military installation; it was a hub for the web of alliances between the French, the Acadian settlers, and the Mi'kmaq people who had inhabited the region for millennia. Father Jean-Louis Le Loutre, a French missionary and agent provocateur, used the fort as a base to coordinate resistance against British expansion, rallying Mi'kmaq warriors and Acadian militia alike. The garrison was small but the strategic stakes were enormous. Whoever held Chignecto controlled the gateway between the French and British empires in North America.

Lieutenant-Colonel Robert Monckton's forces disembarked near the Missaguash River on June 2, 1755. Commander Louis Du Pont Duchambon de Vergor had only 150 soldiers from the Compagnies franches de la Marine and a dozen canonniers-bombardiers to defend the walls. The siege lasted just twelve days. On June 16, a large British bomb punched through the roof of a casemate and killed many of its occupants. Vergor surrendered. The British renamed the fort Cumberland and, the following day, sent 300 men to take Fort Gaspareaux on the far side of the isthmus, which capitulated without a fight. The fall of both forts settled the boundary dispute decisively in Britain's favor and marked a turning point that reverberated far beyond the marshes of Chignecto.

What happened after the fort's surrender was far more consequential than the battle itself. The fall of Beausejour triggered the Expulsion of the Acadians, one of the most devastating forced removals in North American history. British forces began rounding up French settlers across the region, burning their villages behind them to prevent return. Thousands of Acadian families were scattered across the Atlantic seaboard, to France, to Louisiana, to the Caribbean. Their homes, farms, and communities, built over generations, were destroyed. The fort that was supposed to protect them instead became the catalyst for their dispersal. Even after the expulsion, the fort saw conflict. Acadian and Mi'kmaq partisans raided it in 1757, killing and scalping two soldiers. A generation later, during the American Revolution, Patriot sympathizers besieged it again in the Battle of Fort Cumberland of 1776.

Since 1920, the site has been designated as the Fort Beausejour - Fort Cumberland National Historic Site, attracting about 6,000 visitors each year. Portions of the earthworks have been restored, and a museum tells the story of the French, British, Acadian, and Mi'kmaq people whose lives collided on this narrow strip of land. The bastions still trace their pentagonal outline against the sky, overlooking the Tantramar Marshes where the tides of the Bay of Fundy still surge and retreat. From the ramparts, you can see across to where Fort Lawrence once stood, barely a kilometer away. Two nations once faced each other across that distance, and in the space between them, the Acadians lost everything. The marshes have reclaimed much, but the earth still holds the shape of what was built here and what was broken.