Marc will have some input regarding Canadian squabbles I'm sure, but for the US...

I'd take a look at the Indian Wars, both pre- and post-Civil War (say 1840-1860 and 1866-1891, although there is a good case for going into the Colonial problems and those in the East before 1840). Wide ground I know, but there's some interesting stuff in there.

For campaigns: Arizona from 1848 through 1876.
Kansas and Colorado 1866-1872 (roughly)
Texas 1873-1875
Arizona again (1880s)
Montana (1876)

And that's just scratching the surface.