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WASHINGTON — For two decades, Qassem Soleimani methodically built a global web of proxies, militias and allies capable of doing Iran's bidding while operating largely in the shadows. Now, that same network is likely poised to avenge his killing, posing a threat that could strike just about anywhere in the world.

As Iran vowed "harsh retaliation," Soleimani's killing in a U.S. drone strike has ricocheted across the world, triggering embassy security alerts throughout the Middle East and stepped-up security in U.S. cities. As the world braces for a potentially spiraling escalation, U.S. defense officials said another 3,000 troops were headed within hours to the Middle East.

And at the Mar-a-Lago resort, where President Donald Trump was vacationing, the activity level has been intensifying, as officials try to prepare for any potential retaliatory options, a source familiar with events at the Florida club said.

At the time of the drone attack, Soleimani had been "plotting imminent and sinister" attacks against U.S. interests, Trump said from the resort on Friday, in his first public remarks after ordering the airstrike.

"For years, the Islamic Revolutionary Guard Corps and its ruthless Quds Force under Soleimani's leadership has targeted, injured and murdered hundreds of American civilians and servicemen," he said, citing recent attacks on American targets in Iraq, including "rocket strikes that killed an American and injured four American servicemen very badly, as well as a violent assault on our embassy in Baghdad."

Soleimani, the longtime commander of Iran's elite Quds Force and a revered figure in that country, largely engineered Iran's strategy of using asymmetric and paramilitary tactics to overcome its major disadvantage: Its formal military is no match for major fighting forces like the United States.

"What the Iranians have in front of them is a menu of potential routes they could take. It could be directly or indirectly," Naysan Rafati, an Iran analyst at the International Crisis Group, said.

Iran's response could take many forms, targeting U.S. individuals, assets, interests, allies or some combination. It might not be immediate and could play out in the Middle East, elsewhere or in cyberspace.

Nick Rasmussen, former director of the National Counterterrorism Center and an NBC News contributor, said Iran's overseas capabilities can be viewed as three concentric circles.

In its immediate neighborhood, Iran could put together a response almost immediately, such as this week's assault on the U.S. Embassy in Iraq. In the broader Middle East, it might take Iran more time, but its proxies could still threaten U.S. personnel, business people or tourists, Rasmussen said. In the rest of the world, including Southeast Asia and Latin America, Iran's capabilities are less advanced — but not negligible.

"The point is they're not starting from zero almost anywhere. There's always something they have, almost everywhere," Rasmussen said. "It's hard to defend against that because it could be almost anywhere."

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Here are some potential targets for retaliation by Iran:

U.S. military assets

The United States has troops currently staged throughout the Middle East and the Persian Gulf, including in Iraq, Syria, Qatar, Bahrain, Kuwait and the United Arab Emirates.

Since the 1980s, Iran has armed and trained a web of Shiite militias in Iraq, Lebanon, Syria, Yemen and elsewhere, staging lethal attacks on American targets and allies. The proxies allow Iran to exert military and political influence at relatively low cost, while forcing adversaries to think twice before launching a direct attack on Iranian soil.

Iran wields extensive political influence in predominantly Shiite Iraq, where Soleimani's Quds Force built up and armed militias that targeted U.S. troops.

Iran's partners in Iraq could stage attacks on the 5,000 U.S. troops deployed in Iraq, the U.S. Embassy in Baghdad, the American consulate in the northern city of Erbil, Americans working in the oil industry or the hundreds of contractors who support U.S. troops and maintain the Baghdad Embassy compound.

An Iraqi Shiite militia group called Imam Ali Brigades chants slogans against the Islamic State militant group in Tikrit, Iraq, on March 14, 2015. Khalid Mohammed / AP

In Syria, where a small U.S. force is deployed, Iran has a large contingent of Shiite fighters backed by Iranian Revolutionary Guard advisers who could target the American troops or their Kurdish allies.

U.S. embassies

Although heavily fortified in the Middle East, U.S. embassies are a prominent projection of American global power and have been attacked by Iran before.

Only a few days before Soleimani's killing, as U.S.-Iran tensions were escalating, the U.S. Embassy was attacked by pro-Iran protesters in Baghdad — the same city where Soleimani was later struck. Trump on Twitter blamed Iran for "orchestrating" the attack.

The storming of the U.S. Embassy in Tehran in 1979 with Americans taken hostage for more than a year became a powerful symbol of Iran's revolution and the end of U.S.-Iranian diplomatic relations. Several years later, Iran and its proxies were implicated in the bombing of the U.S. Embassy in Kuwait.

Persian Gulf

Apart from attacks by proxies, Iran could use its growing missile and drone arsenal to inflict damage on America's Persian Gulf allies by hitting oil facilities and tankers at sea. If Iran wanted to take more drastic action, it could fulfill its threat to effectively shut down oil shipping through the Strait of Hormuz by attacking tankers and planting mines.

U.S. allies

Iran could hit one of America's regional allies, many of whom happen to be enemies of Iran anyway, such as Israel, Saudi Arabia and the United Arab Emirates.

In Lebanon, Israel's neighbor, Iran helped create the Shiite militia Hezbollah, which has grown into the country's single most powerful military and political force. Hezbollah could launch attacks on American interests in Lebanon or renew its rocket attacks in neighboring Israel.

Supporters of Lebanon's Hezbollah leader Sayyed Hassan Nasrallah gesture as they listen to his televised speech in Baalbeck, Lebanon Oct. 19, 2019. Aziz Taher / Reuters file

In Saudi Arabia, a September 2018 attack on a key hub of the kingdom's oil infrastructure was blamed by the U.S. and European governments on Iran, highlighting Tehran's ability to penetrate Saudi air defenses with precision.

American citizens

Clément Therme, a research fellow at Sciences Po, a Paris-based political science institute, said another potential response was for Iran's proxies to capture Americans overseas.

"The reaction could be hostage taking," Therme said, particularly in places such as Iraq or Afghanistan. In Iraq, the U.S. Embassy has already told all Americans to leave the country immediately in an implicit acknowledgement of that possibility.

In Lebanon, Hezbollah's significant control over the airport and certain regions of the country could create more turf where Americans could be vulnerable, security experts said. And Hezbollah's reach isn't limited to the Middle East: Terrorism analysts have tracked the group's planting of organizational roots across Africa and Latin America, and to a lesser extent in the United States.

Cyberwarfare

Iran will likely employ its growing cyberwarfare prowess to strike at the United States directly, former U.S. officials say. But staging a terrorist attack on U.S. soil presents a tougher challenge for Iran, and it could take some time to organize and plan, former intelligence officials said.

"Iran has really been upping its A-game as far as its cyber capabilities and to be able to use those cyber capabilities to conduct espionage, to conduct intelligence gathering, as well as to conduct mayhem," said Theresa Payton, former White House chief information officer and CEO of the cybersecurity firm Fortalice Solutions.