1. To deter Saudi Arabia from getting nukes, America promises nuclear retaliation against Iran if it were to attack the Saudis. The House of Saud, doubting that Iran will credit American promises and politically reluctant to rely on infidel America, purchases nuclear weapons anyway. A year later, two Saudi weapons are stolen by Islamist soldiers or terrorists. Our intelligence agencies have reason to believe the intention is to smuggle the weapons into America.
Variation: The House of Saud itself falls under the sway of Islamists, who then control Saudi weapons.
2. Fearing intimidation by nuclear Iran, two neighboring Sunni states go nuclear. Unlike the United States and the Soviet Union, the antagonists are so close to each other geographically that missile warning times are in the minutes; in addition, regional early-warning systems are primitive, and Cold War–style controls over weapons release are nonexistent. In the midst of a regional confrontation, one side mistakenly believes the other has launched a preemptive strike (as happened during the Cold War). A fervent local commander promptly launches on warning. The other side retaliates. One million die. Oil production crashes.
Variation: In the melee, missiles pre-programmed for retaliation against Israel are launched. Israel retaliates massively. The Arab world believes Israel started it all.