AMNA NAWAZ: Welcome to the "News Hour."

The tenuous truce between the U.S.

and Iran was tested today, as American ships clearing a lane in the Strait of Hormuz came under fire and sank Iranian boats.

Also today, Iran fired drones and missiles at a key oil terminal in the United Arab Emirates, spooking markets and raising concerns that the war could resume.

Nick Schifrin begins our coverage.

NICK SCHIFRIN: Today, the war in Iran threatened to reignite.

The Islamic Revolutionary Guard Corps, or IRGC, fired missiles and drones and deployed small speedboats like these to attack U.S.

guided missile destroyers trying to clear Iran's choke hold in the Strait of Hormuz.

And the U.S.

fired back, said the military's top Middle East commander in a briefing to reporters.

ADM.

BRAD COOPER, Central Command Commander: The IRGC has launched multiple cruise missiles, drones and small boats at ships we are protecting.

We have defeated each and every one of those threats through the clinical application of defensive munitions.

NICK SCHIFRIN: Those munitions used by jets, ships and more than 15,000 service members conducting Project Freedom.

It's designed to clear a lane through which dozens of ships currently stuck can pass through the strait under a defensive umbrella, which a U.S.

official compared to zone defense, rather than the man-to-man defense of an escort.

ADM.

BRAD COOPER: We have multiple layers that include ships, helicopters, aircraft, airborne early warning, electronic warfare.

We have a much broader defensive package than you would have ever if you were just escorting.

NICK SCHIFRIN: Iran's campaign in the strait has damaged nearly 30 vessels since the start of the war, their charred and rusting remains littering the strait and ensuring Iran's choke hold.

Today, two U.S.

destroyers entered the strait and two U.S.-flagged commercial ships passed safely, the U.S.

goal to convince shipping and insurance companies they too can send their vessels through the strait.

ADM.

BRAD COOPER: Over the last 12 hours, we have reached out to dozens of ships and shipping companies to encourage traffic flow through the strait consistent with the president's intent to help guide ships safely through the narrow trade corridor.

This news has been quite enthusiastically received, and we're already beginning to see movement.

NICK SCHIFRIN: But for the first time since the cease-fire, Iran today resumed its attacks on Gulf oil infrastructure, striking Fujairah, the United Arab Emirates' only major port that bypasses the Strait of Hormuz.

The British military also reported two ships off the coast of the UAE on fire.

And Iran's Foreign Ministry spokesman served this warning to vessels in the strait: ESMAEIL BAGHAEI, Iranian Foreign Ministry Spokesman (through translator): Ships, ship owners and shipping companies are well aware that ensuring their safety and security requires coordination with the relevant authorities of the Islamic Republic of Iran.

NICK SCHIFRIN: As for diplomacy to end the war, this weekend, Iranian state TV released a new 14-point plan, requiring the U.S.

once again to pause any negotiations over its nuclear program and lift its blockade before Iran would release its choke hold over the strait.

This weekend, President Trump wrote: "Iran has not yet paid a big enough price."

And, tonight, following Iran's attacks, the U.S.

military remains poised to resume the war if the president orders them to do so.

President Trump threatened today that if Iran fires on U.S.

vessels -- quote -- "They will be blown off the face of the earth."