He followed up yesterday in a briefing with:
every bridge in Iran will be decimated by 12 o’clock tomorrow night, where every power plant in Iran will be out of business — burning, exploding and never to be used again. I mean, complete demolition by 12 o’clock.
It seems extremely unlikely to me that every power plant and bridge are sufficiently key to Iran's military that they merit destruction, while a threat to destroy all of them is very consistent with economic coercion.
And then today with:
A whole civilization will die tonight, never to be brought back again. ...
This reads to me as very clearly a threat against the civilian population.
After WWII, the US air force investigated itself and determined that the deliberate targeting of civilian housing areas was not a war crime because they contributed to the war effort by being located in Germany and having some war relevant industry or infrastructure in the same city. Independent of this justification, the actual reasoning for the strikes was to deplete enemy morale by killing civilians or at least depriving them of housing.
This has become the founding myth of the US air force, and despite the total lack of success in WWII they have continued to pursue this strategy of depleting morale by bombing civilians in every strategic bombing campaign since. AFAIK no US airman has ever been convicted of war crimes related to bombing.
Trumps tweets don't even explicitly say that the purpose of bombing the infrastructure is to hurt civilians (only the "crazy bastards" in charge of the strait, presumably the government of Iran). That puts them ahead of most past US bombing campaigns.
Last week the US president announced that:
Yesterday morning he posted that:
These are threats to target civilian infrastructure as a coercive measure, which would be a war crime: if Iran doesn't allow tankers through the Strait of Hormuz, the US will cause massive damage to power plants, bridges, and possibly water systems. The US has historically accepted that this is off limits: destroying a bridge to stop it from being used to transport weapons is allowed, but not as retribution or to cause the civilian population to experience "Hell". The Pentagon's own Law of War Manual recognizes this distinction: when NATO destroyed power infrastructure in Kosovo, it was key that the civilian impact was secondary to the military advantage and not the primary purpose. [1][2]
To be clear, what Iran has been doing to precipitate this, by attacking civilian tankers for the economic impacts, is itself a war crime. But that does not change our obligations: the US has worked for decades to build acceptance for the principle that adherence to the Law of War is unconditional. It doesn't matter what our enemies do, we will respect the Law of War "in all circumstances". We've prosecuted our own service members, and enemy combatants, under this principle.
I hope that whatever is said publicly, no one will receive orders to target infrastructure beyond what military necessity demands. You don't need to be a military lawyer (and I'm certainly not one) to see that such orders would meet the threshold at which a member of the armed forces is legally required to disobey. I have immense respect both for commanders who refuse to pass on such orders and for service members who refuse to carry them out. [3]
[1] The manual cites Judith Miller, former DoD General Counsel, writing on Kosovo that "aside from directly damaging the military electrical power infrastructure, NATO wanted the civilian population to experience discomfort, so that the population would pressure Milosevic and the Serbian leadership to accede to UN Security Council Resolution 1244, but the intended effects on the civilian population were secondary to the military advantage gained by attacking the electrical power infrastructure." If the impact on civilians had been the primary motivation for NATO's attacks on power infrastructure they would not have been lawful.
[2] "Military objectives may not be attacked when the expected incidental loss of civilian life, injury to civilians, and damage to civilian objects would be excessive in relation to the concrete and direct military advantage expected to be gained." (DoD LoWM 5.2.2) and "Diminishing the morale of the civilian population and their support for the war effort does not provide a definite military advantage. However, attacks that are otherwise lawful are not rendered unlawful if they happen to result in diminished civilian morale." (DoD LoWM 5.6.7.3)
[3] "Members of the armed forces must refuse to comply with clearly illegal orders to commit law of war violations." (DoD LoWM 18.3.2)