Tensions between the United States Navy and the Iranian Islamic Revolutionary Guard have once again spiked, resulting in the American Navy’s USS Mahan firing three warning shots and deploying a helicopter to intercept four fast-moving Iranian vessels closing with the destroyer and other U.S. ships on Sunday.

According to reports, the USS Mahan contacted the Iranian vessels by radio in the moments leading up to the incident, demanding that they stop or change course as they approached the U.S. naval vessels at a high rate of speed.  After repeated warnings, the destroyer deployed a helicopter to drop a smoke float, or floating smoke grenade, and fired three warning shots from one of its onboard .50 caliber machine guns, at which point the Iranian vessels broke off pursuit.  The Mahan also reportedly utilized flares, bells and whistles in an attempt to warn the Iranian vessels off of their approach.

The incident began as the Mahan, along with the amphibious assault ship USS Makin Island and the replenishment oiler USNS Walter Diehl, entered the Strait of Hormuz, heading toward the Persian Gulf.  An Iranian Navy vessel shadowed the U.S. ships from a safe distance, an action that is considered routine and non-threatening between Navies.  Soon, however, four small attack boats began harassing the U.S. ships, approaching them six times over a nine-hour span, culminating in a final approach that prompted the U.S. Navy to fire the warning shots.

The final of said six approaches, which U.S. Defense Officials have referred to as “unprofessional” was done at approximately fifteen to twenty knots and closed to within five hundred to nine hundred yards of the American vessel.  Coming within five hundred yards of the USS Mahan was seen as dangerous for the ship and those accompanying it, leaving the American Navy with no alternative but to fire warning shots as a means to dissuade further harassment.

This marks the first aggressive interaction between these nations of the new year, but is far from the first such incident between Iran and American forces in recent months.  In November, a US Navy Seahawk helicopter was targeted by an Iranian Revolutionary Guard Corps patrol boat near the same Strait of Horrmuz while escorting the U.S. Aircraft Carrier, USS Dwight D. Eisenhower, which was also traveling to the Persian Gulf.  In September, Iran and U.S. forces were at odds both in the air, where Iran threatened to shoot down two US Navy aircraft as they were flying just inside the strait, and on water, as seven Iranian fast-attack boats were involved in an “unsafe encounter” with the USS Firebolt.

In August of last year, a US Navy Patrol boat was forced to fire three warning shots at an Iranian boat that was “harassing it” and, perhaps worst of all interactions, ten U.S. Navy personnel were captured by Iranian forces after veering off course and suffering a mechanical failure while in Iranian waters.  The incident was later assessed by American officials to have resulted from “failed leadership at multiple levels from the tactical to the operational.”

Incoming American President, Donald Trump addressed these sporadic, but aggressive interactions on the campaign trail, and in no uncertain terms, made it clear that they will not stand once he’s in office.

“And by the way, with Iran, when they circle our beautiful destroyers with their little boats and they make gestures that our people — that they shouldn’t be allowed to make, they will be shot out of the water.”  President Elect Trump announced at a campaign rally last September.