President Trump expressed optimism Monday that the war between Iran and Israel was coming to an end, just hours after Iran launched largely symbolic strikes on a U.S. military base in Qatar that were intercepted.

Experts say the U.S. should remain on alert for Iranian threats, even as a fragile ceasefire takes shape. Iran’s leaders have touted “a variety of options” for getting back at the U.S. for bombing three of its nuclear enrichment facilities on Saturday — and some of them would be much harder to trace than missiles.

“We’ve got 46,000 troops overseas. We’ve got three carrier battle groups going to be out there. There’s lots of asymmetric things that the Iranians can do,” retired Army Brig. Gen. Steve Anderson, who served in Iraq during the 2007 troop surge, said Monday on CNN. 

“They can do cyber warfare against us, they can exercise terrorist attacks. We talked about sleeper cells coming alive here in the United States and, oh, by the way, they can take economic action and just close the Strait of Hormuz,” he added. “So there’s a lot that the Iranians can still do.”

Here’s how Iran could take revenge on the U.S.:

Cyberattacks

Iran, though hamstrung in its military might thanks to a more than two week Israeli air campaign against Tehran, still possesses a sophisticated offensive cyber capability. 

Cliff Steinhauer, director of information security and engagement at the National Cybersecurity Alliance, sees denial-of-service attacks as the most likely option, “where they launch a bunch of traffic toward a government or a contractor’s website.”

Steinhauer told The Hill these attacks would probably be directed at targets without significant cyber defenses, including infrastructure in local municipalities and smaller cities and counties. “You might see some potential activity, bells going off, but maybe not like a full shutdown of a power plant or real manipulation of controls in a water treatment plant.”

“They probably want to scare us more than they want to damage us at this point in terms of cyber capability,” he added. 

Iran is considered one of the top four nation-state adversaries against the U.S., in addition to China, Russia and North Korea — countries that Tehran could also team up with to overcome any gaps in their cyber capabilities.

Tehran has set a precedent for using cyber methods to retaliate against Washington. After the January 2020 U.S. airstrike that killed Maj. Gen. Qassem Soleimani, the head of Iran’s elite Quds Force, the U.S. detected Iranian attempts to target U.S. government agencies and critical infrastructure, according to Steinhauer.

Additional military attacks

Iran is believed to have about half of its original stock of roughly 3,000 ballistic missiles after exchanging fire with Israel over the past two weeks, armaments that can be used against 19 sites across the Middle East, where U.S. forces are located.

The Iranians on multiple occasions warned it would retaliate against American forces in the region should the U.S. enter the war with Israel. 

Tehran appeared to make good on that promise on Monday when it launched multiple missiles at Al Udeid Air Base in Qatar at around 7:30 p.m. But the attacks appeared carefully planned to avoid escalation and caused no reported casualties. 

While this Iranian response appeared largely symbolic, the country could seek to hit at the U.S. in more anomalous ways, such as by targeting American diplomatic, consular or trade missions.

Another option could be to call on its proxy groups across Iraq, Yemen and Syria — militants who have previously launched attacks on American assets in the region.

And in the region’s waterways, Iran could choose to launch so-called swarm attacks on U.S. warships using drones and speed boats equipped with missiles and rockets, a practice that its Revolutionary Guards Corps Navy has practiced exhaustively over the years.

Retired Gen. Frank McKenzie, a former commander of U.S. Central Command, on Sunday said he believed U.S. troops in Iraq and Syria were “certainly vulnerable” to retaliatory attacks by Iran.

Closing the Strait of Hormuz

One option that would have massive global consequences would be to close the Strait of Hormuz, a vital shipping route through which more than 20 percent of the world’s oil supplies pass daily. 

The waterway, which connects the Persian Gulf to the Arabian Sea, is the main channel through which oil and liquefied natural gas from the Middle East are exported to the rest of the world. 

Roughly 20 million barrels of oil pass through the strait every day, according to the U.S. Energy Information Administration.

Choking off the waterway, by possibly laying sea mines or attacking naval and commercial shipping, would have devastating consequences for the global oil market. 

While Iran has not yet actively disrupted the global oil flow, the Iranian Parliament’s national security and foreign policy committee on Sunday proposed that the Strait of Hormuz be closed in response to the U.S. attack, Tehran state media reported.

Sleeper cells

Trump administration officials have warned Iran may try to activate sleeper cells in the U.S. and the West as one method to hit back at Washington.

U.S. Customs and Border Protection (CBP) on Sunday said the threat of sleeper cells in the U.S. has “never been higher,” according to a memo from CBP Commissioner Rodney Scott. 

Scott said “thousands of Iranian nationals have been documented entering the United States illegally and countless more were likely in the known and unknown” ways.

“Though we have not received any specific credible threats to share with you all currently, the threat of sleeper cells or sympathizers acting on their own, or at the behest of Iran, has never been higher,” Scott added, urging CBP personnel to remain “vigilant.”

Leon Panetta, a former Defense secretary and CIA chief during the Obama administration, told CNN on Thursday that sleeper cells were a real concern. 

“They operate that way, not just in the United States, but around the world. They operate with secret cells. They operate on the basis of terrorism and assassination. That’s something Iran is very capable of and has shown it can do that in other areas,” he said.

“So I think the United States has to be very concerned about that kind of effort to raise hell here in the United States.”