In August, at Finland’s request, the Swedish government deployed fighter aircraft and a warship to help patrol Finland’s eastern airspace and the Gulf of Finland against Russian air incursions and drones straying from Ukrainian operations. Critically, the Swedish assets involved were placed under Finnish command and followed Finnish rules of engagement. It was the latest example of how Finland and Sweden, the two newest additions to NATO, have made qualitative leaps in defense coordination.

The Nordic countries have expanded regional collaboration significantly in the past four years. Between them, Denmark, Finland, Iceland, Norway, and Sweden now boast a new long-term defense strategy, a cross-border transit area, frequent joint exercises, and unprecedented levels of command integration.…