As the 29-member NATO military alliance begins its massive “Trident Juncture” exercise on Thursday, the primary focus is on training to defend the alliance’s sea lines of communication, or SLOCs. Specifically, NATO is preparing to fight to keep open the SLOCs between the U.S., Canada, and Europe. These SLOCs would help decide the outcome of any future Russian invasion.

As my annotated map below shows, the SLOCs have four main components (green boxes). First, there’s the Arctic northern sector, which would be crucial for keeping the Russian northern fleet (centered around the northern red box) from surging into the Atlantic and attacking U.S. resupply ships.