The International Boundary is commonly referred to as the world's "longest undefended border," though this is true only in the military sense, as civilian law enforcement is present. It is illegal to cross the border outside border controls, as anyone crossing the border must be checked per immigration[28][29] and customs laws.[30][31] The relatively low level of security measures stands in contrast to that of the United States – Mexico border (one-third length of Canada–U.S. border), which is actively patrolled by U.S. Customs and Border Protection personnel to prevent illegal migration and drug trafficking.