Since President Donald Trump berated America’s closest allies after the G7 summit in Canada this weekend, it’s worth remembering why the Atlantic alliance matters so much — to Europe as well as the United States.

• Ryan Bohl has explained the history: NATO was designed to stifle the motives for war; the EU was meant to kill the means. Encouraging the continent that produced two world wars to fashion swords from plowshares is a terrible idea. If Europe rearms and remains united, it opens the door to transatlantic confrontation. (We’re seeing that now.) If Europe fragments in the process, it means a return to history, with major European states picking fights with one another and eventually drawing outsiders in.
• Shaun Riordan worried that, without the United States, Europe would not be able or willing to stand up to an assertive Russia and an increasingly hostile Turkey.
• Why should America care if Europe is weak? That brings me to a story I wrote last year, in which I argued Trump was ignoring the advice of the great geostrategists: Halford Mackinder, Alfred Thayer Mahan, Nicholas Spykman and Zbigniew Brzezinski. All four recognized that if a power paramount in Eurasia would exercise decisive influence over two of the world’s three most economically productive regions — Western Europe and East Asia — the third, North America, could not be far behind. During the Cold War, the fear was that Soviet Russia could exercise such an influence. Today, it’s China. In particular, its One Belt, One Road initiative. America needs democratic bridgeheads on the world’s largest continent to contain such a threat — or it will inevitably be overwhelmed by it.