Canada

They're actively talking about annexing us. This would be like Poland investing in Panzers in 1938.

Joining a defense initiative with someone who wants to take us over sounds like a bad idea.

But, if it was Europe asking, maybe.

Two years ago. Absolutely.

Now. You have got to be kidding me.

1. Canada needs to be moving away from the US, not towards

2. What does Canada do when the US says it needs to pay for 90% of it to be “fair”?

The "Let's let the radioactive shrapnel fall in Canada before it gets to the States" dome? No thanks.

If Canada is inside the dome, Canadians can lob anything across the border
If they annex Canada, it makes it easier for Canadians lob things over "any" fence

Iron Dome-like defence system is going to be a very expensive with very little return.
US's border is Canada in the north, Mexico in the south and oceans on its sides, Canada and Mexico wouldn't fire any weapons towards US because they're economically tied.
So the only case where Iron Dome-like system is useful is with missiles over the ocean where everyone can see it coming and have time to shoot it down.
Of course this can be useful if US thinks that the enemy will create satellites that can fire missiles

This is entirely a non-starter. We simply do not have the technology to intercept ICBMs on anything but a local scale. Guarding thousands of km of our boarder isn't feasible when you have literal seconds to intercept something that could land minutes away. I mean, ICBMs are hypersonic by definition since they fall at a rate of something like mach 20. It's like trying to stop a bullet by throwing a BB at it. It can only work by placing ICBM interceptor sites every hundred KM or something across the entire boarder.

Imagine funding billion dollar interceptor missiles by the thousands. And that presumes that we have ones that work.

The Iron Dome works because it intercepts rockets that are technologically more similar to WWII Soviet weapons than modern day missiles. Against that, an ICBM is like comparing a hobby rocket to SpaceX's Space Ship One.

Any serious talk about it is simply grifting, and Canada shouldn't take part unless if the other side could provide evidence of it working without the projected costs being greater than our entire GDP. And even then, who are we going to use it against? The Russians? All evidence points to them not being able to get their existing ICBMs to work, let alone make new ones. And Canada isn't in between China and the US, so that's not a threat to us at all. And that presumes that China can lob an ICBM at several times the distance Russian needs to.

And all this presumes that we can trust anything that's said regarding any kind of friendly cooperative from the south. First try ending your damn trade war with us, then we can start talking about some smaller low stakes cooperatives before ramping things up to things that has the potential to cripple our entire economy and autonomy.