Trump wants the Navy to destroy Iranian gunboats that harass US ships. Here's why it doesn't do that.

• President Donald Trump tweeted Wednesday that he has instructed the US Navy to 'destroy' any and all harassing Iranian gunboats.
• "You just don't get to shoot at people for being jackasses," retired US Navy Capt. Rick Hoffman, the former commanding officer of a cruiser, told Insider.
• Military legal experts told Insider that the lawful use of force must be based on the perceiving hostile intent — a condition that Trump's order does not take into account.
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Iranian gunboats have a long history of sailing out and harassing US Navy ships, but the service's go-to move is not to respond by blowing them out of the water.

President Donald Trump said Wednesday that he has ordered the US Navy to "destroy" Iranian gunboats that "harass our ships at sea."

Last week, a swarm of nearly a dozen Iranian gunboats "conducted dangerous and harassing approaches" against US Navy and Coast Guard vessels, US Naval Forces Central Command revealed.

The incident did not escalate to the US of lethal force.

"You just don't get to shoot at people for being jackasses," retired US Navy Capt. Rick Hoffman, who commanded a cruiser, told Insider.

The rules of engagement for the US military are classified, but Hoffman's understanding is that aggressive action can only be taken in instances where hostile intent is perceived by the commanding officer.

"They have to demonstrate hostile intent or conduct a hostile act for us to be able to shoot them under the rules of engagement," he explained. "That's the bottom line."

Hoffman told Insider that while "worrisome maneuvers, harassing behaviors, disregarding the bridge-to-bridge calls are all annoying," such actions are not inherently a threat to a warship and do not necessarily constitute hostile intent.

Iran's small boats are manned by hardliners who often drive fast and recklessly around US warships in contested areas, but they are not typically a danger to the crews of the larger American naval vessels.

Military legal experts also said that "hostile intent" is key to decisions on the use of force.

"Rules of engagement provide for a captain of a ship to use force if in fact there is evidence of hostile intent toward his ship and his crew," retired US Army Maj. Gen. John Altenburg Jr., who has decades of experience with Military Law, told Insider.

"The ship's captain always has the right to defend the ship," he explained. Acknowledging that the situation is complicated, he said that "what the president said probably goes beyond what the rules of engagement say."

Trump's stated order to the Navy appears to eliminate considerations of hostile intent from the equation, as it calls for the destruction of any Iranian gunboat engaging in harassment.

There is also the question of how the president's tweet factors into international law.

Whether or not using lethal force in response to harassment is legal "depends on the nature of the harassment," Eugene Fidell, who served in the US Coast Guard and currently teaches military law at Yale Law School, told Insider.

On one hand, "a warship has a right to defend herself from unprovoked attack," he explained. "On the other hand, the use of force must be reasonable and proportionate, so an ex ante order to 'destroy,' regardless of the nature and severity of the actual threat, goes beyond the bounds of international law."

Vice Chairman of the Joint Chiefs of Staff Gen. John Hyten said at the Pentagon Wednesday the president's tweet was clear, explaining that "we, as the military, have to apply the clear direction from the commander-in-chief into lawful orders that we execute."

"What the president said sends a great message to Iran," the general said. "That's perfect. We know how to translate that into our rules of engagement. We don't talk about rules of engagement, but they're based on the inherent right of self defense."