The Ottawa Citizen reported today that Canada and the United States are in the final stages of negotiating a "new Canada-U.S. military agreement that will set down each country's roles and responsibilities should North America be attacked". The details are scant, but it would appear that part of the agreement will permit the military of one state to conduct military operations on the territory of another, subject presumably to ground rules and consent.
The legal issues raised by cross-border deployment are undoubtedly complex. Here are a few that come to mind. First, how would an American deployment in Canada that involves the use of force be treated in Canadian criminal law? As discussed in NSL, p.174 et seq, the Criminal Code is woefully silent on the relationship between domestic criminal law and the use of (lethal) force to stave off an armed attack. That silence, applied to the actions of an allied army on Canadian territory could be the source of some unease.
Conventional status of forces agreements, such as the NATO SOFA, may constitute at least a partial response. These agreements give concurrent criminal jurisdiction to the sending and receiving states, and primacy to the former where the purported criminal conduct stems from "offences arising out of any act or omission done in the performance of official duty". Consider, though, a repeat of the friendly fire incident involving a US air force pilot that took the lives of several CF members in Afghanistan in the early 2000s. That hugely controversial case was prosecuted under US military law. If the same events were to recur during a combined operation in Canada, it can be imagined that the pressures for a Canadian proceeding would be enormous.
A second thorny question stems from Canada's different relationship with international law. For one thing, Canada is a party to the Ottawa treaty banning land mines, while the United States is not. An allied force laying mines in Canada obviously creates legal complications.
Perhaps even more concerning is Canada's membership in the International Criminal Court. Famously, the United States has been animated in denying that Court any sort of jurisdiction over US personnel. And yet, the International Criminal Court would have jurisdiction over, e.g., war crimes taking place on Canadian territory, regardless of the nationality of the accused. A US military deployment in response to an armed attack would be subject to this jurisdiction, no doubt to the disgruntlement of the United States. Exactly how this particular dilemma would be resolved is unclear -- but one thing Canada would have to avoid is a "carve out" purporting to extend to US forces immunity from ICC jurisdiction while deployed in Canada. Canada has staved off such a bilateral immunity agreement in the past. Not least, there is a strongly held view in the international legal community that such a carve-out would place Canada in non-compliance with it own international obligations.
We must wait and see what the promised new US-Canada arrangement says on these and other issues.