In a much-discussed essay, the historian Robert Kagan recently depicted President Obama as presiding over an inward turn by the United States that threatened the global order and broke with more than 70 years of American presidents and precedence. He called for Mr. Obama to resist a popular pull toward making the United States a nation without larger responsibilities, and to reassume the more muscular approach to the world out of vogue in Washington since the war in Iraq drained the country of its appetite for intervention.

The New Republic cover article, “Superpowers Don’t Get to Retire,” struck such a nerve in the White House that many in the foreign policy establishment considered part of Mr. Obama’s speech last month at West Point outlining a narrower vision for American force in world affairs to be a rebuttal, and the president even invited Mr. Kagan to lunch to compare world views. But the rapid advancement of militants from Iraq and Syria on Baghdad, and Mr. Obama’s announcement on Friday that he was weighing the use of force to counter them, makes the debate suddenly less abstract.

To Mr. Kagan, American action to stop the militants is imperative, but a continued military presence in Iraq and action in Syria would have averted the crisis. “It’s striking how two policies driven by the same desire to avoid the use of a military power are now converging to create this burgeoning disaster,” Mr. Kagan said in an interview...