NOBODY can defend against a nuclear attack. The US has been working on missile defence for nearly 50 years, but have still not succeeded. 
In order to be an actual threat to Canada, terrorists would have to hit us with several 9/11 scale or larger attacks. Terrorism is a crime problem, not a threat to national security, and the RCMP deals with it quite effectively, even if they have to create it so they have something to deal with. 
Economic sabotage happens; our financial institutions are strong and our government stable, and so those things are dealt with without fuss or significant damage.