The US embassy in Ottawa has had a heavy marine presence for quite some time.
Three years ago I was house hunting there, and one house viewed, had been rented by the US Gov't. for the corps. They were moving out as they were vastly increasing their manpower in preparation for the move into the new embassy. They had the chain of command from the President on down to their unit on a kitchen cupboard.
Interestingly they have to rent, as to buy would require a bill to pass congress with the add on's from both sides. So the state dept. just expenses it through rent.
Likewise with SE's comments I don't see the need for customs agents at the airport to be armed. The Canadian federal government should have more visible mounties there in my view.