When President Obama wrote last month to Iran ’s supreme leader, Ayatollah Ali Khamenei , urging him to overcome a decade of mistrust and negotiate a deal limiting Tehran’s nuclear ambitions, it was perhaps the president’s last effort to reach a reconciliation with Iran that could remake the Middle East. Today, Mr. Obama needs a foreign policy accomplishment more than ever, and he sees time running out on his hope of changing the calculus in a Middle East where Americans are, against his instincts, back on the ground. But the forces arrayed against a deal are formidable — not just Mr. Khamenei and the country’s hard-liners, but newly empowered Republicans, some of his fellow Democrats, and many of the United States’ closest allies. As negotiators head back to Vienna this week for what they hope will be the final round of talks, Mr. Obama’s top national security advisers […]