NORAD is unique - it is a joint command. The NORAD agreement requires the commander and deputy to from different countries (practically, it's always been a American commander and Canadian deputy). The agreement also requires integrated and binational staff. NORAD itself is responsible for aerospace warning, and aerospace control. It exercises aerospace control by having operational control (though not ownership) of national units.

When the article talks about Canadian officers serving as deputy corps commanders, that's referring to Canadian participants in exchange programs. The US military has exchange programs which do 1:1 exchanges with allied forces. For example, a Canadian might end up as as a US deputy division or corps officer, and an American might end up as a Canadian brigade staff officer or deputy commander. The exchange programs are part of a broader American program to build military-military ties, increase interoperability, and maybe grant unique perspectives.

NORAD spots must be filled, and will be filled. It just means that you're probably not getting the best or most motivated candidates anymore. The exchange slots will probably whittle away.

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