Which is, in the end, the most disturbing aspect of Mr. Obama’s performance. Throughout his presidency, he has excelled at explaining what the United States cannot do and cannot afford, and his remarks Thursday were no exception. “Ukraine is not a member of NATO,” he said. “We don’t have those treaty obligations with Ukraine.” If Iraq doesn’t form an acceptable government, it’s “unrealistic” to think the United States can defeat the Islamic State.

Allies are vital; the United States overstretched in the Bush years; it can’t solve every problem. All true. But it’s also true that none of the basic challenges to world order can be met without U.S. leadership: not Russia’s aggression, not the Islamic State’s expansion, not Iran’s nuclear ambition nor China’s territorial bullying. Each demands a different policy response, with military action and deterrence only two tools in a basket that includes diplomatic and economic measures. It’s time Mr. Obama started emphasizing what the United States can do instead of what it cannot.