The United States is sending a clear message to Iran’s navy: stop acting like pirates or face the consequences. To make sure its message hits home with Iran, the U.S. military is taking the unprecedented step of offering to station Marines on foreign merchant vessels. It’s the latest sign the U.S. is still very much committed to establishing safe sea routes in the Middle East.

Forces from the Bataan Amphibious Ready Group (ARG) and 26th Marine Expeditionary Unit (MEU) arrived in the Middle East this week. The ARG/MEU joins a host of other U.S. military assets in the region, all meant to deter Iran from harassing or seizing any more merchant vessels.

In the last two years alone, there were at least 20 instances of Iranian vessels firing on, harassing, or taking foreign ships and vessels sailing the Strait of Hormuz. Around 20% of the world’s crude oil passes through the narrow naval chokepoint.

Some of the Marines from the 26th MEU were flown to Bahrain ahead of time.

A U.S. official told ABC News the Marines were there to train in anticipation of being placed aboard commercial ships transiting the region. The official said the Marines’ mission would be to defend the ships, but that they’d have the right to defend themselves as necessary.

There are a host of bureaucratic hoops to jump through before United States Marines are likely to be seen on foreign ships, though. The Marines and Navy sailors would only provide the security at the request of the ships involved. Also, any deployment of U.S. forces on foreign vessels would require the approval of the country for which the ship is sailing, as well as the owner of the ship itself.

John Kirby, the White House National Security Council spokesman, said, “The Strait of Hormuz is a vital seaway that has a huge impact on seaborne trade around the world. It’s a critical chokepoint in the maritime world, and we have seen threats by Iran to affect that chokepoint.”

Iranian armed forces spokesperson Brig. Gen. Abolfazl Shekarchi said the U.S. military isn’t needed in the region.

Iranian media quoted Shekarchi as saying, “What do the Persian Gulf, the Gulf of Oman and the Indian Ocean have to do with America? What is your business being here?”

In addition to 3,000 U.S. Navy sailors and Marines, the ARG/MEU brings with it the dock landing ship USS Carter Hall and the amphibious assault ship USS Bataan, which can carry a couple dozen rotary and fixed-wing aircraft. Dock landing ships like the Carter Hall can also carry aircraft, but perhaps their biggest asset is the ability to carry tactical and amphibious combat vehicles.

In its buildup to deter Iran, the U.S. previously sent a guided missile destroyer to the region in addition to fighter jets like F-16s, F-35s, and even some A-10 Thunderbolts, more affectionately known as “Warthogs.”

While they made a name for themselves busting tanks on land, the Pentagon thinks Warthogs could offer a nice countermeasure to the go-fast boats Iran’s navy likes to use while harassing ships.

Iran is also trying to beef up its presence in the Persian Gulf and Strait of Hormuz. Iranian state media reported the Revolutionary Guards’ navy was outfitted with a host of drones and missiles with a 600-mile range.

The Associated Press contributed to this report.