The border between the US and Canada began with the Treaty of Paris in 1783. The treaty also ended the war between Great Britain and the separating colonies that later formed the United States. The border that was established, called the 45th parallel, began between the Lower Canada and New York State. The Great Lakes and the St. Lawrence River became the boundary between Upper Canada and the United States.
The international boundary is commonly known as the world’s longest undefended border. It is true, but only in the sense of military defense, because civilian law enforcement is present. The lack of security on the US-Canada border is in stark contrast to the US-Mexico border, which is a third of the length as the US-Canada border.
As of December 2010, Canada and the United States are negotiating an agreement titled "Beyond the Border: A Shared Vision for Perimeter Security and Competitiveness" which would give the U.S. more influence over Canada's border security and immigration controls, and more information would be shared by Canada with the U.S.