The Northamerican Aerospace Defense Command, a joint Canadian-US military organization, has been in existance for about the last 45 years. It has target location equipment (radar) and interdiction assets (fighter aircraft) from both nations, and was the primary line of defense during the cold war. According to the NORAD web site, they have assumed a new mission since September 11, 2001 of tracking possible terrorist threats.

In other words, tell them that Canada is and has been actively involved in the defense of the US, and to get stuffed.