As Lord Ismay, NATO’s first secretary general, somewhat cheekily observed, the trans-Atlantic alliance was created to “keep the Soviet Union out, the Americans in and the Germans down.” Seven decades later, those goals have largely been met (yes, the Germans have risen, but in the right ways), and many people — including, evidently, the president of the United States — wonder whether the alliance still has a purpose.

It does. It remains the most successful military alliance in history, the anchor of an American-led and American-financed peace that fostered Western prosperity and prevented new world wars. No one has proposed anything credible to improve upon it. But as the allies gather in Brussels this week for their annual meeting, many are wondering whether the American president is intent on wrecking it.

Born after World War II, NATO linked America and Europe not just in a mutual defense pledge but in advancing democratic governance, the rule of law, civil and human rights, and an increasingly open international economy.

The alliance was the core of an American-led liberal world order that extended to Asia and relied on a web of international institutions, including the United Nations and the World Bank.