The White House accused the Iranian authorities of "maiming and killing Americans, acting as the primary state sponsor of terror and brutally murdering its own people for merely speaking out against its oppressive rule" for the past 47 years.
Trump, it insisted, was "making the entire region safer and more stable by eliminating Iran's short- and long-term threats to the United States and our allies".
The war is reported to have claimed thousands of lives:
The US-based Human Rights Activists News Agency says that 1,606 civilians, including at least 244 children, have been killed in Iran since the start of the conflict
Lebanon's health ministry has said 1,345 people have been killed in Israeli strikes since 2 March
Missile attacks on Israel launched from Iran and Lebanon have killed 19 civilians since the start of the war, according to Israeli emergency services
In the Gulf states that have come under Iranian attack, local authorities have reported at least 24 people killed, most of them security personnel or foreign workers - including 12 in the UAE, seven in Kuwait, two each in Oman, Saudi Arabia and Bahrain, and one in Bahrain
Tom Fletcher, the UN's humanitarian chief, told the BBC Radio 4's Today programme on Friday that "somewhere along the way" international law had been "thrown aside".
"The rules are very clear and very strong," he added, but the problem was "enforcement". He described the war as "reckless".
The experts' letter also highlights the attack on a primary school in the Iranian town of Minab on the war's first day, reported to have killed at least 168 people including 110 children.
The US Department of Defense has said it was investigating the attack, which a growing body of evidence has suggested was probably the result of a US strike.
One theory believed to be part of the investigation is that the school, next to an Islamic Revolutionary Guards Corps base, could have been hit as a result of outdated intelligence.
The experts' letter says the strike "likely violates international humanitarian law, and if evidence is found that those responsible were reckless, it could also be a war crime".
The letter has been published in Just Security, external, an online journal based at New York University School of Law.