If Mr. Bush had decided to display American leadership and exercise American power by launching a diplomatic campaign against Iran rather than a military one against Iraq 10 years ago, the United States’ international standing would be far greater today.

The Bush administration’s decision to go after Iraq rather than Iran was a fatal one, and the long-term consequences are only now becoming clear, namely a devastating American failure in the battle to prevent a nuclear Iran, reflected in Washington’s willingness to sign a deeply flawed agreement.

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.