Mistakes happen. Generally though, a nation doesn’t accidentally build a fort on the wrong side of an international border. This kind of incursion would be viewed rather unfavorably by the neighboring country even if they were friendly to each other. Throw in a history of mutual mistrust and territorial incursions and it could get a lot worse. The United States made just such an incursion onto British territory in Lower Canada in 1816 when it began construction of a defensive fort at the northern tip of Lake Champlain.
The 1783 Treaty of Paris formally ended the American Revolutionary War between the United States and Great Britain, with Britain recognizing the independence of its former colonies. The treaty also established an initial border that would be further defined and refined by subsequent treaties. It followed 45° north in some portions, separating the U.S. states of Vermont and northeastern New York from Britain’s Lower Canadian territory.
…thence down along the middle of that [Connecticut] river to the forty-fifth degree of north latitude; from thence by a line due west on said latitude until it strikes the river Iroquois or Cataraquy…
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Fort Blunder was located on the knob protruding into the waters of Lake Champlain in the satellite image above. The green arrow rests on the 45th line of latitude. The border between the United States and Canada runs north of the 45th parallel and Fort Blunder, so what happened?
Waterways served as vital highways connecting interior towns to eastern coastal cities in North America during a period when overland roads consisted of little more than rutted, muddy tracks. Lake Champlain experienced ferocious naval battles during the Revolution and again in the War of 1812 over its control. It would be pretty simple for British forces to sail from nearby Montréal for example, and head directly over the border into Lake Champlain to capture strategic trading ports should hostilities ever resume. Recent history demonstrated that American had reasons to feel concerned.
The United States selected Sand Island as an optimal defensive point. It was situated where Lake Champlain emptied into the Richelieu River, where it narrows to about a mile across.(1) A fort placed here could challenge approaching vessels and command access to the lake and to its interior watershed.
Indeed, if it were actually located in the United States.
The Twelve Mile Circle often mentions the difficulties faced by eighteenth and early nineteenth century surveyors. They labored over difficult wilderness terrain with crude instruments. Minor errors were not only a possibility but a probability. The United States genuinely thought it was building a fort within its territory.
However a more precise survey conducted jointly by the United States and Great Britain in 1819 proved that the selected spot stood about three quarters of a mile north of the 45th parallel. Construction halted immediately. Local residents dubbed the unnamed fortification "Fort Blunder" and proceeded to scavenge much of the stone and brickwork to build homes in nearby Rouses Point over the next couple of decades.
The blunder was repaired in the Webster-Ashburton Treaty of 1842. The United States and Great Britain negotiated a number of tradeoffs to better define a border. One of their deals nudged the boundary line through Lake Champlain just enough north to place the site within the United States. The current border as defined by the International Boundary Commission contains a number of these adjustments, deviating slightly from 45° north in both directions, from 44° 59’ 28.77” to 45° 00’ 59.84.” Monument 645, due north of where Fort Blunder once stood, marks 45° 00’ 37.95.”
The United States built a new fortification at the same location named Fort Montgomery. It never saw action and the army abandoned it as the twentieth century dawned. Masonry fortifications had long since become obsolete due to the invention of rifled cannons and with improvements in the overland road infrastructure. Many of its stones were used in the foundation of a bridge crossing Lake Champlain from Rouses Point to Alburg, part of a Great Depression era Works Project Administration venture. What little remains of Fort Montgomery today rests private property as it continues to crumble into the earth with little chance of restoration.
But at least it’s now in the United States.
(1) Coincidentally the other bank in Vermont contains a fascinating geo-oddity.