RESOLUTE BAY, Canada — After finishing a training drill on surviving the bitter cold, the soldiers gathered around Ranger Debbie Iqaluk to hear about an inescapable fact of life in the high Arctic: The ice is melting despite the frigid temperatures.

And that means the Russians are coming.

Her retelling of how she watched as an enormous iceberg fractured, just a few feet from the military base here, was riveting. It is one thing to be told constantly that the melting polar ice cap has opened up the Arctic, disappearing what used to be an impenetrable barrier between North America and Russia. It is quite another to see it firsthand.

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The iceberg took five years to melt, but by 2018 it was gone, taken over by a sea that with each year is melting earlier in the season. That has brought Russia right to Canada’s doorstep, cutting into the “Fortress North America” concept that has long comforted military planners on this side of the Atlantic Ocean.

NATO is rushing to try to catch up. Last month, hundreds of troops from member countries and partners, including France, Norway, Finland and Sweden, joined Canadian soldiers, reservists and rangers for the Nanook-Nunalivut exercises that aimed in part to help alliance forces match Russian readiness in extreme-cold climes. (The United States sent observers but no troops this year.)