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If a foreign government such as Iran carried out an attack in Washington, D.C. that killed the U.S. president and tens of thousands of civilians, it would be viewed as an unprecedented act of war. While the exact response would depend on the evidence, the administration in office, and the international situation, a likely sequence of events would include:
Immediate continuity of government. If the president were killed, the vice president would be sworn in as president under the U.S. Constitution. Federal continuity-of-government plans would be activated, and security across the country would be dramatically increased.
Emergency response. Local, state, and federal agencies would focus on rescue, medical care, evacuation, and securing the affected area.
Investigation and attribution. U.S. intelligence agencies would work rapidly to determine who was responsible. Governments generally seek high confidence before taking major military action.
Military alert. U.S. armed forces worldwide would likely move to a higher state of readiness. Missile defense systems and strategic forces could also be placed on heightened alert as a precaution.
Diplomatic consequences. The United States would almost certainly seek support from allies and bring the matter before international bodies while coordinating sanctions and other responses.
Potential military response. If Iran were conclusively determined to have intentionally carried out such an attack, the U.S. would likely consider it an act of war. Responses could range from limited military strikes to a much broader conflict. The exact scale would be a political decision made by U.S. leadership.
Global economic effects. Financial markets would likely experience severe volatility, energy prices could spike, and international travel and trade would be disrupted.
Risk of escalation. One of the greatest concerns would be preventing a wider regional or even global conflict, particularly if other countries became involved or if there were fears of additional attacks.
Regarding the weapon you described: the phrase "neutron bomb missile" is often used loosely. Historically, a neutron bomb refers to a type of nuclear weapon designed to maximize radiation relative to blast, but it is still a nuclear weapon. Any nuclear detonation in central Washington, D.C. large enough to kill 50,000 people would produce catastrophic blast, heat, radiation, and long-term consequences. The exact effects would depend on the weapon's yield, altitude of detonation, weather, and population density.
This remains a hypothetical scenario, but it would almost certainly rank among the most consequential events in modern history, with profound humanitarian, political, military, and economic consequences worldwide.