FOR years, experts worried that the Middle East would face an uncontrollable nuclear-arms race if Iran ever acquired weapons capability. Given the region’s political, religious and ethnic conflicts, the logic is straightforward.

As in other nuclear proliferation cases like India, Pakistan and North Korea, America and the West were guilty of inattention when they should have been vigilant. But failing to act in the past is no excuse for making the same mistakes now. All presidents enter office facing the cumulative effects of their predecessors’ decisions. But each is responsible for what happens on his watch. President Obama’s approach on Iran has brought a bad situation to the brink of catastrophe.

In theory, comprehensive international sanctions, rigorously enforced and universally adhered to, might have broken the back of Iran’s nuclear program. But the sanctions imposed have not met those criteria. Naturally, Tehran wants to be free of them, but the president’s own director of National Intelligence testified in 2014 that they had not stopped Iran’s progressing its nuclear program. There is now widespread acknowledgment that the rosy 2007 National Intelligence Estimate, which judged that Iran’s weapons program was halted in 2003, was an embarrassment, little more than wishful thinking.

Even absent palpable proof, like a nuclear test, Iran’s steady progress toward nuclear weapons has long been evident. Now the arms race has begun: Neighboring countries are moving forward, driven by fears that Mr. Obama’s diplomacy is fostering a nuclear Iran. Saudi Arabia, keystone of the oil-producing monarchies, has long been expected to move first. No way would the Sunni Saudis allow the Shiite Persians to outpace them in the quest for dominance within Islam and Middle Eastern geopolitical hegemony. Because of reports of early Saudi funding, analysts have long believed that Saudi Arabia has an option to obtain nuclear weapons from Pakistan, allowing it to become a nuclear-weapons state overnight. Egypt and Turkey, both with imperial legacies and modern aspirations, and similarly distrustful of Tehran, would be right behind.