The Rush–Bagot Treaty of 1817 provided a plan for demilitarizing the two combatant sides in the War of 1812 and also laid out preliminary principles for drawing a border between British North America and the United States.

The Treaty of 1818 saw the expansion of both British North America and the United States, with their boundary extending westward along the 49th parallel, from the Northwest Angle at Lake of the Woods to the Rocky Mountains.

The treaty redefined the border between New Hampshire, Vermont, and New York on the one hand, and the Province of Canada on the other, resolving the Indian Stream dispute and the Fort Blunder dilemma at the outlet to Lake Champlain.

After a significant portion of the construction was completed, measurements revealed that at that point, the actual 45th parallel was three-quarters of a mile (1.2 km) south of the surveyed line.

Archibald Campbell led the way for the United States, while Donald Cameron, supported by chief astronomer Samuel Anderson, headed the British team.

In mid-July, the Canadian government announced that fully vaccinated American citizens and permanent residents could visit Canada starting August 9.

[29] Business advocacy groups, noting the substantial economic impact of the closure on both sides of the border, called for more nuanced restrictions in place of the blanket ban on non-essential travel.

[31] Beyond the closure itself, US President Donald Trump also initially suggested the idea of deploying United States military personnel near the border with Canada in connection with the pandemic.

[32][33] The International Boundary is commonly said to be the world's "longest undefended border", though this is true only in the military sense, as civilian law enforcement is present.

The border also runs through the middle of the Mohawk Nation at Akwesasne and even divides some buildings found in communities in New England and Quebec.

[39] Following the September 11 attacks in the United States, security along the border was dramatically tightened by the two countries in both populated and rural areas.

[43][44][45][46] In August 2020, the United States constructed 3.8 km (2.4 mi) of short cable fencing along the border between Abbotsford, British Columbia, and Whatcom County, Washington.

[47] Before 2007, American and Canadian citizens were only required to produce a birth certificate and driver's license/government-issued identification card when crossing the Canada–United States border.

[48] Since June 2009, every traveler arriving via a land or sea port-of-entry (including ferries) has been required to present one of the above forms of identification to enter the United States.

[52] American permanent residents who are NEXUS members also require Electronic Travel Authorization when crossing the Canadian border.

[52] Smuggling of alcoholic beverages ("rum running") was widespread during the 1920s, when Prohibition was in effect nationally in the United States and parts of Canada.

[60][61] From the beginning of January 2017 up until the end of March 2018, the RCMP intercepted 25,645 people crossing the border into Canada from an unauthorized point of entry.

The Canadian territory of Yukon shares its entire western border with the U.S. state of Alaska, beginning at the Beaufort Sea at 69°39′N 141°00′W / 69.650°N 141.000°W / 69.650; -141.000 and proceeding southwards along the 141st meridian west.

After making a sharp turn eastbound, the border follows this parallel across the Tsawwassen Peninsula, separating Point Roberts, Washington, from Delta, British Columbia, and continues into Alberta.

From here, it proceeds eastward through the Angle Inlet into the Lake of the Woods, turning southward at 49°19′N 94°48′W / 49.317°N 94.800°W / 49.317; -94.800 (near Dawson Township, Ontario) where it continues into the Rainy River.

The border then crosses the Height of Land Portage over the divide between the Hudson Bay drainage basin, and that of the Great Lakes.

At 45°59′N 83°26′W / 45.983°N 83.433°W / 45.983; -83.433 (between Drummond Township, Michigan, to the west and Cockburn Island (Ontario) to the east), the border turns southward into the False Detour Channel, from which it reaches Lake Huron.

It heads southward to 45°56′N 67°47′W / 45.933°N 67.783°W / 45.933; -67.783 (near Amity, Maine), from whence it follows the Monument Brook further south into the Chiputneticook Lakes, which subsequently leads the border to the St. Croix River.

Such airports were built before the U.S. entry into World War II as a way to legally transfer U.S.-built aircraft, such as the Lockheed Hudson, to Canada under the provisions of the Lend-Lease Act.

The runway is entirely within North Dakota, but a ramp extends across the border to allow aircraft to access Canadian customs.

The Fourth Connecticut Lake Trail (New Hampshire/Quebec) crosses several times while following the border vista before heading back to the United States.

At Vancouver's Pacific Central Station, passengers are required to pass through U.S. partial pre-clearance and pass their baggage through an X-ray machine before being allowed to board the Seattle-bound Amtrak Cascades train, which makes no further stops before crossing the border at Blaine, Washington, where the train stops for another CBP inspection.

Seasonal vessel inspection stations are operated at tourist destinations such as Heart Island, New York, and Rockport, Ontario.

The island is part of Charlotte County, New Brunswick, but is physically connected by the Franklin Delano Roosevelt Bridge with Lubec, Maine, the easternmost tip of the continental United States.

Alaska is a non-contiguous U.S. state bounded by the Bering Sea; the Arctic and Pacific oceans; and Canada's British Columbia and Yukon Territory.