I believe prompt global strike (and similar schemes) to be highly unstabalizing and to be avoided at all costs. The global suspicion (with a country with 2000k+ nukes none the less) would be suffocating. If the USA employed such a system, the moment you acted as anything less than a white knight, the world be up in arms. And justly. Your friends (the UK and Canada and Australia) would start edging away. And then you're truly fucked. (Canadian).
According to that logic, Canada should already have that attitude to the US, since most of their population is inside aircraft range...
Airplanes [edit: and rocket artilleri, for that matter] in range is functionally the same thing as such a strike capacity, but cheaper.
There is no reaction from Canada and won't be; see the democratic peace theory.
ICBMs are too expensive to use to deliver conventional explosives or kinetic kill projectiles. Forget bioweapons and toxins, they're not any deadlier than cluster bombs.
The power of an explosion follows an inverse square law, so you can destroy something with 1/4 the explosive if you can get twice as close to it.
If you could drop thousands of 25 lb bomb equivalent devices an hour with GPS accuracy at a city, you could wreck it's infrastructure in a few weeks.