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.

Then there are the Baltic Sea defensive interests (the two overlapping green boxes), which would deny Russian offensive operations into Western Europe and against Scandinavia's southern flank. These SLOCs would also afford NATO the means of surrounding Russia's Kaliningrad garrison (southernmost red box). Finally, there's the SLOC that would allow NATO to harass Russian forces operating between Estonia and Finland (the most easterly green box), and from moving to reinforce Kaliningrad.

The centerpiece of Trident Juncture work is the U.S. Navy's Harry S. Truman carrier strike group, which will operate in the northern sector. The at-sea command entity for fleet operations, Truman's crew will seek to orchestrate allied maritime and air operations to protect the Atlantic. A key focus here will be training to deny Russian fighter-bomber and submarine forces from punching their way through the Norwegian Sea. While it won't be seen, attack submarines and NATO's broader anti-submarine warfare capabilities will also feature heavily here.

Trident Juncture also carries political-strategic importance in reflecting the non-NATO member Scandinavian nations of Finland and Sweden's increasing cooperation with NATO. In a striking rebuke of Vladimir Putin's increasing threats against their interests, both nations are boosting their NATO interoperability. And as this Finnish military video illustrates, they are doing so with a pride that echoes Finland's heroic resistance to the Soviet invasion of 1939. This historic alignment formalizes the failure of Putin to cow his neighbors into submission.

NATO member Norway remains the anchor nation for Trident Juncture. Evidencing as much in recent months, we've seen increased British and Norwegian training of U.S. Army and Marine forces in arctic ground combat. Norway will also host many of the air force squadrons participating in this exercise.

To be sure, Russia is furious about Trident Juncture. NATO, Putin says, is escalating tensions for no reason other than a base Russophobia. But Putin is wrong and Russia's anger is misguided. This exercise is simply an overdue response to escalating Russian aggression. Trident Juncture is defensive in nature, countering Russia's threat to Europe and America. That's a good thing, as is the possibility that the exercises will serve as a warning to China to reconsider its growing cooperation with Putin.