America’s withdrawal from the Iran deal and relocation of its Israeli embassy to Jerusalem send an unmistakable signal about the emerging Trump foreign policy: The administration wants to enlarge American power rather than adjust to decline. For now at least, the Middle East is the centerpiece of this new assertiveness.

For President Obama, Iran’s rise was an unavoidable fact. Confronting Iran meant risking a war even bigger and uglier than the one in Iraq. Mr. Obama wasn’t only personally opposed to such a war, he believed that neither Congress nor public opinion would sustain it. The era in which the U.S. could dominate the Middle East was over; the wisest course was to negotiate an arrangement that would protect core U.S. interests and cover for an American withdrawal. 

The Iran deal, President Obama and his supporters believe, accomplished all that and more. By taking the nuclear issue off the table, at least for the time being, the agreement averted the danger of a U.S.-Iranian military confrontation. Moreover, it weakened hard-liners inside Iran by undermining their core argument that Iran faced an external threat requiring permanent social mobilization even as it strengthened moderates by tying the country ever more closely to the world economy. If supported by the West, the Obama administration believed, moderates would gradually consign the Islamists to the political fringes.

From this perspective, the deal was a masterstroke of diplomacy. Its supporters now fear that Iranian and American hard-liners, energized by the failure of their more accommodating rivals, will steer the countries toward a policy of confrontation ending in war—and that the result of this war will be to accelerate rather than retard American decline in the Middle East and beyond. 

President Trump’s approach is different. His instincts tell him that most Americans are anything but eager for a “post-American” world. Mr. Trump’s supporters don’t want long wars, but neither are they amenable to a stoic acceptance of national decline. As to the wisdom of accommodating Iran, Team Trump believes that empowering Iran is more likely to strengthen the hard-liners than the moderates. As Franklin Roosevelt once put it in a fireside chat, “No man can tame a tiger into a kitten by stroking it.” ...