askchapo

This is a very broad question, it doesn't have a simple answer.

There are a lot of possible air based threats, and you can bet that desk jockeys in their war departments spend all day dreaming up things that they need to defended against. But to defend against everything would bankrupt your nation, and so they all have to make decisions about what sort of things to invest into. When there's no restraint, you get insane shit like Golden Dome, which is envisioned to be a world-wide space based missile defense system that can shoot down all sorts of stuff.

The case of Israel is informative of how difficult it is to address all possible types of threats. They have to maintain two separate types of missile defense systems (Iron Dome and David's Sling) because the requirements for intercepting short range rockets vs long range rockets is so different. Despite USA support, Israel's military budget is not literally unlimited so they have to pick and choose carefully based on what they anticipate the actual threats to be. And it's still expensive as hell and not super reliable.

Readiness is also an important factor - it's expensive to keep any defense system in a state of active alert, ready to act. An example of that would be on 9/11; the National Guard had the capability to shoot down airliners, but the readiness of the weapons to be used didn't give them enough time to respond (the F-16s that were scrambled to intercept Flight 93 could not be loaded with gun ammo or missiles in time).

To your specific question about a ballistic missile launched at Ottawa: Canada doesn't have anything that can defend against that. They have extensive early warning radar systems, and so could give advanced warning if it came from a direction that was actively monitored. But the best they could do is inform Ottowans to seek shelter a few minutes ahead of time. If for some reason there was advanced intelligence of a ballistic missile attack on Ottawa, then an arrangement with the USA could probably be made to station a THAAD battery there... but that's a lot of eventualities lining up.

Or is it just one of those fun illusions of safety that could be broken by a sufficiently determined group?

Pretty much. Soft targets are the rule, not the exception. But the hardest way to defend against an attack of any type is to repel it with force while it's happening. That's a major reason why nations engage in diplomacy, alliances, and intelligence gathering... and invest relatively little in things like missile interceptors. You only do that if you're expecting to be attacked.