The Strait of Hormuz, the world’s most sensitive oil-transportation choke point, remains in focus, after U.S. and Iranian officials on Thursday said Iran shot down a U.S. military drone near the waterway.

“This event is raising fears Iran and the U.S. are headed toward a military confrontation that would have serious price consequences for energy. There is too much dangerous activity going on in the Strait of Hormuz,” said Phil Flynn, senior market analyst at Price Futures Group, in a note.

It is the latest in a string of incidents near the waterway that have contributed to rising fears of a U.S.-Iran military conflict that could disrupt the flow of crude oil from the Middle East. It comes less than a week after a pair of oil tankers were attacked near the coast of Iran which the U.S. blamed on Tehran.

Here’s a look at the Strait of Hormuz and why it is so important to the global crude-oil market.

Where is the Strait of Hormuz?

The Strait of Hormuz is a narrow waterway that links the Persian Gulf with the Gulf of Oman and the Arabian Sea.

At its narrowest point, the waterway is only 21 miles wide, and the width of the shipping lane in either direction is just 2 miles, separated by a two-mile buffer zone.

Why is it important?

Oil tankers carrying crude from ports on the Persian Gulf must pass through the strait. Around 18.5 million barrels a day of crude and refined products moved through it in 2016, nearly a third of all seaborne-traded oil and almost 20% of all crude produced globally, according to the U.S. Energy Information Administration. That makes the Strait of Hormuz the world’s most sensitive oil transportation choke point.

What’s the latest?

President Donald Trump, in a tweet, said, “Iran made a very big mistake!”

Asked if the U.S. planned a retaliatory strike, Trump told reporters, “You will soon find out.” But Trump added that he thought the hit may have been a mistake on the part of Iranian military personnel, saying it may have been the fault of someone “loose and stupid.”

Iran’s Islamic Revolutionary Guard Corps, a paramilitary that answers solely to Supreme Leader Ayatollah Ali Khameini, said it shot down the drone when it entered Iranian airspace near the Kouhmobarak district in southern Iran’s Hormozgan Province, the Associated Press reported. Kouhmobarak is around 1,200 kilometers southeast of Tehran and near the Strait of Hormuz. The U.S. military confirmed an unmanned drone was shot down by an Iranian surface-to-air missile, The Wall Street Journal reported (paywall).

“This was an unprovoked attack on a U.S. surveillance asset in international airspace,” said Navy Capt. Bill Urban, spokesman for U.S. Central Command.

The downing of the drone and last week’s tanker attacks follow a May incident that saw four tankers damaged in what Gulf authorities deemed “sabotage.” The incidents follow the U.S. decision to reimpose sanctions on Iran over its nuclear program, with Washington moving this spring to end waivers for several importers of Iranian crude. Military bases housing U.S. forces in Iraq have also come under rocket attack in the past week

Oil futures, which were already set to get a lift from the Federal Reserve’s signal that it is prepared to deliver a rate cut as early as next month, rallied sharply to set June highs. Brent crude BRNQ19, +3.87% the global benchmark, was up around 4% at $64.24 a barrel, while the U.S. standard-bearer, West Texas Intermediate crude CLQ19, +5.47% was more than 5% higher at $56.77 a barrel.

Stock-market investors initially shrugged off the incident, with the S&P 500 SPX, +0.61% rising to an all-time high Thursday in a rally attributed to the Fed’s dovish tilt on rates, while the Dow Jones Industrial Average DJIA, +0.64% DJIA, +0.64% advanced more than 200 points. But equities trimmed gains after Trump’s latest remarks, with the S&P holding a 0.4% gain, while the Dow was up 0.5%, or 122 points.

Read: Escalation in Mideast oil attacks could $7 per barrel to price

The U.S. in May announced it was sending an aircraft carrier group, bombers and a Patriot antimissile battery to counter what the Trump administration said were “clear indications” that Iran and its proxies were preparing to possibly attack U.S. forces in the region. That was in addition to the existing presence of the Bahrain-based U.S. Fifth Fleet.

Could Iran close the strait?

The Fifth Fleet’s presence has long cast doubt on Iran’s ability to close the waterway, analysts said.

The U.S. naval presence would make it extremely difficult for Iran to choke off traffic, but the country “has the strategic depth to stage one-off attacks on vessels, not just in the critical chokepoints but also in the region’s relatively open waters,” Helima Croft, global head of commodity strategy at RBC Capital Markets said, in a May research note.

Is war likely?

Iran’s leadership appears comfortable taking large risks in its engagements, targeting U.S. forces directly rather than sticking to deniable, low-level attacks against third-party targets, said Henry Rome, analyst at Eurasia Group, in a note. The death of U.S. service members would be almost certain to force Trump’s hand, he said.

The U.S., however, appears unlikely to retaliate militarily for the downing of the drone, he said.

“Today’s attack underscores our decision to raise the probability of war between the U.S. and Iran to 40%, but war is still not base case,” said Henry Rome, analyst at Eurasia Group, in a note. The political-risk consulting firm earlier this week raised its estimate of the probability of a U.S.-Iran military confrontation over the next six months to 40% from 30%.