According the the New York Times (NYT), the US Constitution says Congress has the power to declare war while it makes the president the commander in chief of the military so he can take decisions about bombing once a war has been declared. And since the US Congress has not declared war on Iran, his bombing would seem to be illegal.

The NYT also points out that the Constitution makes ratified treaties the supreme law of the land. One of these ratified treaties is the United Nations Charter which the Senate ratified after World War II. The Charter says that, except in self-defence, a country may only attack another country if the U.N. Security Council has authorized it which in this case it had not.

So in theory the bombing was illegal. But in practice since the end of the Cold War Presidents from both parties have carried out airstrikes and other limited interventions without asking the Congress. For example President Bill Clinton participated in NATO air wars over Kosovo and over President Barack Obama did the same over Libya. And Trump attacked Syrian government forces during his first presidency.

CNN reports that the Trump administration is relying on the president’s authority under Article II of the Constitution which says he has power to direct US military forces in engagements necessary to advance American national interests abroad. The White House counsel’s office and the Justice Department were both involved in the legal analysis for the strikes. The administration relied, in part, on memos about war powers written by the DOJ’s Office of Legal Counsel under previous administrations of both parties.

A senior Justice Department official said if this conflict continues for an extended period, the administration may have to go to Congress for approval, but maintained that “bombing three nuclear sites” does not rise to the level of needing congressional approval. The official also noted the Trump administration has the support of senior House and Senate leaders.

For more analysis from the NYT see this page and from CNN see this page.