Initial action: back up existing NATO allies and ensure Canada can continually do so in the future

 The first thing is to ensure Canada can get more troops to Europe, and to protect the shipping routes so Canada can continually send troops to Europe.  Since protecting the supply lines is more important, as it ensures a continual commitment to NATO, I cover that first.

Protecting supply convoys to NATO:

Based on this map, there appears to be four choke points where the Russian navy, and its submarines, must pass through to intercept reinforcements shipped into NATO from North America (paths marked in yellow):

So the choke points are quite obvious from above, but I marked them in green to make it extremely obvious as where Canada and the US need to position their ships for monitoring Russian submarines and intercepting Russian warships:

The four choke points in my opinion are:

1)  From Greenland to Iceland.

2)  From Iceland to Finland.

3)  From Norway/Finland to Denmark/Poland.

4)  From Greece to Turkey.

Covering off these choke points would be crucial to preventing Russian submarines from sinking supply ships from North America.

Last I will mention that it may be possible to send Russian submarines through the Arctic circle, then south between Greenland and Canada to intercept North American shipping, but I think it may be too risky with icebergs and too much trouble to get back to Russian naval bases that way.  I am guessing they want to stay as close to Russia as possible so they can quickly return to base to rearm and refuel.

Is this necessary?  Well, we want to discourage further aggression in Europe, as they are the only countries that can afford goods produced by North American workers, who are amongst the most expensive in the world.