Iran vowed revenge after the U.S. assassinated a top commander in the Islamic Revolutionary Guards Corps.
Major-General Qassem Soleimani was the chief architect of Iran's strategic alliances, and a web of battle-hardened fighting forces - which he helped build - stand ready to enact retribution.
Iran could strike back at U.S. facilities in Iraq, through what are known as the Popular Mobilization Forces or PMF.
This coalition of predominantly Iraqi Shiite paramilitary fighters include units that fought house-to-house to push Islamic State out of Iraq.
Washington blames elements of the PMF for rocket attacks on U.S. bases which killed an American contractor.
U.S. airstrikes on PMF leaders prompted the militia to lay siege to the U.S. embassy last week.
A senior PMF commander was killed in the same airstrike that took out Soleimani in Baghdad Thursday night.
Other deadly Iranian proxies include Hezbollah, in Lebanon.
That group has amassed an arsenal of rockets that it could use to target Israel, provoking the American ally into armed conflict as it did in 2006.
It was a Hezbollah suicide bomber who in 1983 attacked a U.S. Marine barracks in Beirut, killing 241 American personnel.
Its reach could also extend beyond the Middle East: Argentina accuses Hezbollah, with the assistance of Iran, of the 1994 bombing of Buenos Aires Jewish center that claimed 85 lives.
Iran could use cyberattacks to strike at the U.S., aimed at infrastructure and more. In 2016 the Department of Justice accused seven Iranians of hacking American banks and a New York dam.
Tehran could also target the global oil supply in the Persian Gulf. Saudi Arabia and Washington blame Iran for airstrikes on a Saudi oil facility last September.
The attack shut down more than five percent of world oil supply.
It's unclear how Iran will respond but it seems less a matter of if than a question of where, and when.