Important note first:
This is a completely hypothetical scenario. According to ᴀssessments by the US, IAEA, and Western intelligence, by March 2026, Iran will not possess operational nuclear weapons (not exceeding the threshold for manufacturing warheads). Iran also does not have intercontinental ballistic missiles (ICBMs) capable of reaching US territory. Therefore, the likelihood of this actually happening is almost zero.

Below is a realistic timeline of what would happen if Iran were capable of launching a nuclear warhead targeting the US (based on the publicly available US nuclear program):
0–3 minutes: Immediate detection

SBIRS (Space-Based Infrared System) satellites and NORAD/STRATCOM ground radar would immediately detect the launch.
The early warning system identifies this as a ballistic missile and calculates its trajectory within 90 seconds.

Nuclear alarm sounds at the underground command centers (Cheyenne Mountain, Offutt AFB).

3–8 minutes: Confirmation & President Notification

NORAD confirms this is a real nuclear threat.

The “Nuclear Football” (nuclear briefcase) is delivered to President Trump (or his successor) within 4–6 minutes.

The President receives an Attack ᴀssessment summary and nuclear response options from STRATCOM.

8–15 minutes: President’s Decision

The President has approximately 6–10 minutes to decide (launch under attack or wait for further confirmation).

Available options:

No immediate response (highly unlikely).

Order the launch of Minuteman III or Trident II missiles from a submarine.

The order is given for B-2/B-52 bombers to be on standby.

15–25 minutes: US activates retaliatory strike.

If the President orders it, the US will immediately launch hundreds of nuclear warheads from silos, submarines, and aircraft.
US missiles would reach Iran in just 25–30 minutes (from submarines, which are closer) to 35 minutes (from continental silos).

25–30 minutes: End of the first phase.

Iran could be destroyed by hundreds of US warheads (SIOP/Oplan 8010).

The entire world enters DEFCON 1 (full-scale nuclear war).

The US Global Missile Defense (GMD) system would attempt to intercept, but its effectiveness would only be about 50–60% against a single target.

Expert ᴀssessment

CSIS & RAND (2025–2026 reports): Iran currently lacks the capability to fire nuclear weapons at the US. This scenario is only possible in 5–10 years if Iran develops ICBMs.

STRATCOM: “The US maintains a policy of immediate and overwhelming nuclear response.”

Conclusion: This scenario is unrealistic at the present stage. Iran is using conventional missiles and drones, not nuclear weapons. The US still maintains an absolute nuclear advantage.