US–Canada International Boundary: History, Disputes & More

October 30, 2018
Marissa Wood

The US–Canada border is the longest international land boundary in the world. Originally established by the 1783 Treaty of Paris and revised nearly a dozen times since, the International Boundary, as it is officially termed, comprises 5,450 total miles and touches eight provinces and 13 states. But scale alone doesn’t begin to describe this curious and sometimes confusing border.

Map showing the land boundary between Canada and the US
The US–Canada border has a rich, storied history.

More than 8,000 monuments and reference points and 1,000 survey control sites – maintained by the International Boundary Commission – dot the United States–Canada border. The boundary can be legally crossed via 119 official border crossing sites, six unstaffed road crossings, 13 international ferries and 39 rail crossings, as well as by air and sea.

United States–Canada Border History

The history of the US–Canada boundary is complex and long-reaching. Formally, it extends back to the very beginning of the United States as an independent nation.


Map of the United States of America Agreeable to the Peace of 1783 from the Library of Congress.

The first formal iteration of the International Boundary was established in 1783’s Definitive Treaty of Peace and Friendship between His Britannic Majesty and the United States of America, which ended the American Revolutionary War. The line between the new nation and Great Britain’s remaining North American colonies – which did not address territory west of the Great Lakes in detail – ran from “the North West Angle of Nova Scotia” to “the Northwesternmost Head of Connecticut River; Thence down along the Middle of that River to the forty fifth Degree of North Latitude.” The 45th Parallel was used as the dividing line west to the Iroquois River, and then roughly divided the Great Lakes.


The Jay Treaty created a boundary commission to better locate the boundary in the St. Croix River.


On 24 December 1814, the Treaty of Ghent formally ended the War of 1812. Among its provisions, the treaty restored the territorial status quo antebellum and called for the establishment of a demarcation commission to finalize issues related to the nascent boundary. The treaty is memorialized today by the Peace Arch, situated near the westernmost point of the US–Canada border in the contiguous United States.


The Treaty of 20 October 1818 – also known as the “Convention Respecting Fisheries, Boundary, and the Restoration of Slaves,” and the Convention of 1818 – addressed US fishing rights along the Newfoundland and Labrador coasts and provided for joint control of land in the “Oregon Country” for 10 years. Perhaps most critically, the treaty first set what would become a major piece of the US–Canada boundary along the 49th Parallel.


Did you know that Alaska was actually a part of Russia before it was purchased by the United States in 1867? The first international boundary established between Alaska and Canada was actually between Russia and the United Kingdom in 1825. This boundary delimitation would be accepted and confirmed by the US when it gained control of the territory in 1867.

Map showing the land boundary between Canada and Alaska
The Canada–United States (Alaska) land boundary has been fully demarcated even though it passes through some very cold, remote mountains.


The Webster-Ashburton Treaty of 9 Aug 1842, resolved the Aroostook War, a slight conflict sparked by significant disputes over the proper location of the international boundary between Maine and the British colony of New Brunswick. The 1842 Treaty also provided additional clarification on the boundary in the Great Lakes region and reaffirmed the 49th Parallel as the international boundary up to the Rocky Mountains.


The 49th Parallel was enshrined as the international boundary west of the Rockies by the Oregon Boundary Treaty of 1846. The agreement declared that “the line of boundary between the territories of the United States and those of her Britannic Majesty shall be continued westward along the said forty-ninth parallel of north latitude to the middle of the channel, which separates the continent from Vancouver’s Island, and thence southerly through the middle of the said channel, and of Fuca’s straits to the Pacific Ocean.” The Treaty, which also declared “free and open” navigation of the western waterways for ships of both nations, was the first to extend the US–Canada border from the Atlantic Ocean all the way to the Pacific Ocean.

For some interesting Canada–USA boundary oddities that were created by the Oregon Treaty check out this blog on Point Roberts!


The United States purchases Alaska from Russia for $7 million in a deal that is often called “Seward’s Folly,” after the US administrator who made the purchase. The old boundary between Russia and the UK is confirmed.


Following the Canadian Confederation of 1867, the 1873 Protocol on the Canal de Haro established a firm boundary in the Haro Strait between San Juan Island (US) and Vancouver Island (Canada).


In 1903, an international tribunal settled a dispute between the US and Canada over the boundary between Alaska and British Columbia in the Alaskan Panhandle. The two States would confirm the demarcation of the boundary following the Award in 1905.

For more about the boundary between Canada and Alaska check out our boundary page!


1908 saw two major milestones in the history of the border between the United States and Canada: the establishment of the International Boundary Commission and the US–UK Treaty of 1908, which called for new surveying between the St. Lawrence River and the Great Lakes, in addition to several minor boundary tweaks. A year later, the Boundary Waters Treaty of 1909 was adopted to prevent and resolve disputes over boundary waters, defined as the “main shore to main shore of the lakes and rivers and connecting waterways, or the portions thereof, along which the international boundary between the United States and the Dominion of Canada passes.”


In 1910, the Passamaquoddy Bay Treaty established an international boundary in Passamaquoddy Bay between Moose Island and Treat Island (US) and Campobello Island (Canada).


The 1925 “Treaty between Canada and the United States of America to Define More Accurately and to Complete the International Boundary between the Two Countries” finally codified large sections of the US–Canada boundary and formally established the International Boundary Commission.


The Atlantic Ocean maritime boundary between Canada and the United States has been and continues to be a contentious area. In 1984, after a series of failed negotiations, the International Court of Justice issued a Judgment in the Case Concerning Delimitation of the Maritime Boundary in the Gulf of Maine Area, which defined a partial maritime boundary between the two countries. Questions of sovereignty for Machias Seal Island were not addressed by the Court, and it remains in dispute (see below). 

The 1984 Canada–United States Atlantic Ocean maritime boundary, which has some remaining areas of dispute and contention.

Canada–United States Boundary Disputes

The United States, United Kingdom and Canada have engaged in numerous boundary disputes – largely peacefully resolved – since 1783. Four problem areas remain today.

Most notable among these is the controversy over Machias Seal Island. The sovereignty of the island, located about 10 miles off the coast of Maine in the Gulf of Maine’s “grey zone,” is disputed due to ambiguities in past treaties. The dispute is particularly heated given the area’s prolific lobster fishery, which has been open to Canadian lobstermen (by Canadian edict) since 2002. A Canadian lighthouse has continuously operated on the island since 1832.

Machias Seal Island has been disputed along the U.S.-Canada border for years.
Canada maintains a lighthouse on Machias Seal Island.

At the end of the land boundary in the Strait of Juan de Fuca, Canada and the United States have different interpretation of how the Pacific Ocean maritime boundary should continue westward. The two countries have differing equidistance claims, developed by utilizing different basepoints, resulting in slight differences between their sovereignty claims.

Parts of the Beaufort Sea, off the northern coast of Alaska and Yukon, are claimed by both nations as part of their exclusive economic zone (EEZ). The dispute exists in part because of differing interpretations related to the translations of the 1825 Anglo–Russian Convention.

Precise maritime boundaries around the Dixon Entrance between Alaska and British Columbia are in dispute as well. An “A-B Line” established in the 1903 Alaska Boundary Treaty is considered to be an international maritime boundary by Canada, but not the United States. The US, meanwhile, claims that equidistance is the correct arbiter of sovereignty in the area.

While not directly a matter of international boundaries, the countries differ in their interpretation of the waters of the Northwest Passage. Canada considers it “internal waters,” and therefore completely under the sovereignty of Canada, while the US regards it as an “international strait,” meaning that vessels of all nationalities should have free passage.

Status of the United States–Canada Border

The Canada–United States International Boundary is an undefended border, but it is illegal to cross the border outside of specified areas of control. Due to the good relations between Canada and the United States, there are large areas without fences or other controls. You can find information on crossing the US–Canada border here.

Numerous communities, a country club, a library and even several “line houses” are divided by the US–Canada border.

Check out some of our additional reports on the Canada–United States land boundary, the Alaska frontier, or the maritime boundary!

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