http://lewrockwell.com/north/north1117.html
Inaugural Meeting of North American Defence Ministers Joint Statement
http://www.defense.gov/releases/release.aspx?releaseid=15141
“By virtue of our geography, our peoples, and our trading relationship, our three nations share many defense interests. Threats to North America and the hemisphere are increasingly complex and require non-traditional responses. Building upon the trilateral collaboration under the North American Leaders Summit process, we share a determination to enhance our common understanding of those threats and of the approaches needed to address them.
Could someone tell me who/what is so strong as to be a threat to ALL THREE nations? 
casual reading:
http://en.wikipedia.org/wiki/Project_Camelot
People.
They consider people a threat.