I'm listening to this
Canadian podcast episode about the unlikely but now possible event of a US invasion of Canada. It is, as you'd expect, really grim. The biggest problem to resistance is that they'd get no support from neighbors. The Viet Cong had North Vietnam, and the Taliban had Pakistan and a whole bunch of other sources of support.
(I thought maybe the IRA had a similar situation with support, but I looked at the
Wikipedia page, and holy crap is that complex. I have no idea what went on there.)