Why the U.S. is sending 2,500 marines to the Gulf — and what they might do there

While U.S. President Donald Trump tries to convince allies to help reopen the Strait of Hormuz, he is also sending 2,500 marines to the region — the first deployment of U.S. troops since it and Israel first attacked Iran on Feb. 28.

The deployment is "a major military operation," said Mark Cancian, a senior adviser at the Washington-based Center for Strategic and International Studies, who spent three decades in the U.S. Marine Corps.

Cancian says the U.S. appears to have recognized that "bombing alone" can't entirely protect commercial ships from Iranian drones and missiles, and that it should have had troops ready sooner.

Washington "should have had the marines started before the war, so they would be there now, rather than waiting two-plus weeks," he told CBC News. The marines are expected a few weeks from now.

Iran's ability to choke off traffic through the strait, a conduit for a fifth of the world's oil and liquefied natural gas, has rattled energy markets and helped drive oil prices above $100 US a barrel. 

The troops are part of a rapid-response force based in Japan, Cancian said. The unit includes roughly 2,500 marines, built around an infantry battalion supported by artillery, vehicles and logistics units.

Once they arrive, the marines could be used in several ways — but Cancian says their most likely mission would be helping to reopen the strait, perhaps by seizing small islands near the shipping lanes and setting up air-defence systems.

"That creates a dome or bubble over a certain part of the strait which helps the convoys defend themselves," he said.

The effort would likely involve multiple branches of the U.S. military.

"It would be navy ships escorting convoys, marines on the ground and aircraft flying cover overhead," plus a cyber aspect, Cancian said.

Another possibility would be to seize Kharg Island, an Iranian oil hub which the U.S. struck over the weekend.

Cancian says taking the island could be "a major bargaining chip," but that the marines are more likely to focus on the Strait of Hormuz. Iran has vowed to retaliate for the Kharg attack by striking U.S. sites throughout the United Arab Emirates.

Even a large military effort may struggle to fully secure the strait, which is just about 33 kilometres wide at its narrowest point — forcing ships to pass close to Iranian territory.

Iranian forces can take advantage of nearby islands and mountainous coastline to launch attacks, and Iran has a range of relatively inexpensive tools that can threaten shipping, including drones, small attack boats and naval mines.