Mr. Bush’s responsibility for the disaster now unfolding is twofold: He failed to target Iran a decade ago, and created a climate that made it very difficult to target Iran today. The Bush administration didn’t initiate a political-economic siege on Iran when it was weak, and Mr. Bush weakened America by exhausting its economic power and military might in a futile war. By the time American resolve was needed to fend off a genuine global threat, the necessary determination was no longer there. It had been wasted on the wrong cause.

The correct way to confront the Iranian threat would have been to establish a broad coalition including Russia, the European Union, Sunni Arab countries, Israel and the United States. This would have placed Iran’s leaders in a real stranglehold and forced them to abandon their nuclear project — just as Libya did in 2003.

The Republican Party could have done that in 2003 or 2005 or 2007. But Republican leaders squandered the opportunity. Worse still, the United States got bogged down in Iraq and Afghanistan and that sucked all the oxygen out of America’s lungs. Mr. Bush passed on to Mr. Obama a nation that had lost much of the resolve it had possessed. When faced with a real threat to world peace, America’s will was spent. It had evaporated in the violent streets of Basra and Baghdad.

Sure, Mr. Obama has made mistakes, too. After coming to office, he wasted time on a futile policy of engagement and then on ineffective sanctions. He ignored the British, French, Israelis, Egyptians and Saudis who warned him that he was being naïve and turned his back on the freedom-seeking Iranian masses in June 2009. When Mr. Obama finally endorsed assertive diplomacy and punitive sanctions in 2011 and 2012, it was too little, too late.

But Mr. Obama was operating within the smoky ruins of the strategic disaster he had inherited.

After Iraq, America is a traumatized nation, with a limited attention span for problems in the Middle East. The empire is weary. It has lost the ardor and wisdom needed to deal with the cruelest of the world’s regions and with the most dangerous of the world’s evil powers.

The Geneva agreement being negotiated is an illusion. The so-called moderate president of Iran, Hassan Rouhani, is an illusion, too. So is the hope that Iran’s supreme leader can be appeased. Because America missed the opportunity for assertive diplomacy, all the options now left on the table are dire ones.

Rather than pursuing a dangerous interim agreement, the West must insist that all the centrifuges in Iran stop spinning while a final agreement is negotiated. President Obama was right to demand a settlement freeze in the West Bank in 2009. Now he must demand a total centrifuge freeze in Iran.