This entry is from Wikipedia: Canada-United States Border
Security: Law enforcement approach
The International Boundary is commonly referred to as the world's longest undefended border, but this is true only in the military sense, as civilian law enforcement is present. The relatively low level of security measures stands in contrast to that of the United States – Mexico border (one-third as long as the Canada–U.S. border), which is actively patrolled by U.S. customs and immigration personnel to prevent illegal migration and drug trafficking.
Parts of the International Boundary cross through mountainous terrain or heavily forested areas, but significant portions also cross remote prairie farmland and the Great Lakes and Saint Lawrence River, in addition to the maritime components of the boundary at the Atlantic, Pacific, and Arctic oceans. The border also runs through the middle of the Akwesasne Nation and even divides some buildings found in communities in Vermont and Quebec whose construction pre-dated the border's delineation. On the Maine-New Brunswick border, it divides a golf course.
The actual number of U.S. and Canadian border security personnel is classified; there are in excess of 17,000 United States Border Patrol personnel on the Mexico–U.S. border alone.
Following the September 11, 2001, terrorist attacks in the United States, border security along the International Boundary was dramatically tightened by both nations in both populated and rural areas. Both nations are also actively involved in detailed and extensive tactical and strategic intelligence sharing. It is a common misconception in the U.S. that the nineteen terrorists involved in the September 11 attacks entered the United States via the Canadian border.