The isthmus stretches roughly from Dieppe, New Brunswick, in the north to Amherst, Nova Scotia, in the south. At its narrowest, between Amherst and Tidnish, it measures just twenty-four kilometers. On the Bay of Fundy side, the landscape is dominated by the Tantramar Marshes and a series of prominent ridges -- the Fort Lawrence Ridge, the Aulac Ridge, the Sackville Ridge, the Memramcook Ridge -- that rise above the waterlogged lowlands like the spines of half-submerged creatures. The Northumberland Strait shoreline is different: largely forested, with tidal estuaries winding deep inland. Dikes built by Acadian settlers centuries ago still hold back the ocean from much of the low-lying land, though those dikes are now vulnerable to fifty-year storms, and climate change is eroding the margin further. Nova Scotia and New Brunswick have taken the federal government to court over who should pay to reinforce them.

The isthmus has been fought over with a persistence that seems almost compulsive. During King George's War in the 1740s, the French used Chignecto as a staging area for raids on British Nova Scotia. In 1750, Father Le Loutre ordered the Acadian village of Beaubassin burned to deny it to advancing British troops. The British built Fort Lawrence; the French built Fort Beausejour across the Missaguash River. Five years of armed standoff followed until a British force of more than 2,000 soldiers from Boston took Fort Beausejour in June 1755, an event that triggered the Expulsion of the Acadians. During the American Revolution, Jonathan Eddy's rebel militia besieged the renamed Fort Cumberland in 1776 and failed. Each conflict left its scars in the earth -- earthworks, graves, burned foundations -- layered atop one another like archaeological strata of stubbornness.

As early as 1685, French intendant Jacques de Meulles noted that a simple ditch could create a portage across the isthmus. Canal proposals appeared regularly after 1822 but never secured financing. In the 1880s, engineer Henry Ketchum attempted something bolder: a ship railway that would carry entire vessels on cradles across seventeen miles of track between the Bay of Fundy and the Northumberland Strait. It was three-quarters complete when the Panic of 1890 destroyed its financial backers. A 1930s canal commission concluded the project was economically unviable. A 1960s study tried to revive the idea. None succeeded. The isthmus, it seems, resists being cut.

What the isthmus could not be cut through, it was crossed over. In 1872, the Intercolonial Railway built a mainline between Halifax and Moncton across the southern portion, skirting the Bay of Fundy through the Tantramar Marshes. A branch line ran from Sackville to Cape Tormentine beginning in 1886, connecting to an iceboat service and later a rail ferry to Prince Edward Island. In the early 1960s, the Trans-Canada Highway was routed across the isthmus, and in the 1990s it was upgraded to a four-lane expressway. Route 16 originally led to the Cape Tormentine ferry terminal but was modified in 1997 to connect with the Confederation Bridge at Cape Jourimain. Today, every car, truck, and train traveling between Nova Scotia and the rest of Canada crosses this same narrow corridor -- a funnel through which an entire province's connection to the continent must pass.

The modern isthmus faces a threat older than any army. The dikes that Acadian settlers built to reclaim marshland from the sea still protect much of the low-lying terrain, but they were designed for a climate that no longer exists. Rising seas and intensifying storms are pushing the engineering margins past their limits. If the dikes fail, the Trans-Canada Highway, the railway, and the fiber-optic cables that carry Nova Scotia's internet traffic could all be severed. The isthmus would not quite become ocean -- not yet -- but it could become impassable. The debate over who pays for reinforcement has become a legal and political battle of its own, with the provinces arguing it is a matter of national importance and the federal government treating it as regional disaster mitigation. Four centuries after the first European noted how easy it would be to cut a ditch across this land, the question has reversed itself: how hard will it be to keep the ditch from cutting itself?