I think you've addressed an interesting point. Although the security and prosperity partnership has the opportunity to work within the NAFTA area--that is, Canada, the U.S., and Mexico--it is possible that any two jurisdictions can work. If they are more aligned or closer, they can work together. There isn't the need to have all three at once. I think this allows a leadership role in taking into effect ones that are already available and ready to go on.
On May 1st, 2007. See this statement in context.