Syria's Assad regime and its Iran-backed allies have pledged to retaliate against any U.S. attack, and analysts say weapons and terror networks at their disposal mean the threats should be taken seriously.
"Iran is a huge threat," said Ariel Cohen, a senior research fellow at the Heritage Foundation who specializes in Russia and Central Asia.
Both Iran and Syria have threatened to retaliate against Israel and other U.S. allies in the Middle East in the event of a U.S. attack on Syria over its alleged use of chemical weapons against civilians. Hundreds of Syrians in a region held by rebels were reported killed in the Aug. 21 attack.
Iran's ruling mullahs are Syria's main ally in the region and view the survival of the Assad regime as important to their aims. Supreme Leader Ayatollah Ali Khamenei-confidante Mehdi Taeb said that Syria is Iran's "35th province...if we lose Syria we won't be able to hold Tehran," writes Karim Sadjapour, an Iran expert at the Carnegie Endowment for International Peace.
Syria provides Iran a port on the Mediterranean and a transit to Iran's terrorist proxy in Lebanon, Hezbollah. Iran is suspected of helping Syria evade U.S. and European sanctions by selling its oil on the international market, according to a report from Reuters.
If Iran decides to retaliate against U.S. allies to protect Assad it could go after not only Israel but the Arab states allied with the U.S. such as Saudi Arabia, the United Arab Emirates and Jordan.
Iran could order Hezbollah and its Iranian Revolutionary Guard Corps to use their networks around the world to target several countries that support the military strike. Hezbollah has killed hundreds of civilians, including Americans, and it depends on Syria as a transit for weapons and occasional safe haven.
Iran could also try to close the Straits of Hormuz, the narrow Persian Gulf passage for about 20% of the world's petroleum supply, Cohen says.
"When we keep an eye on Syria we need to keep an eye on the security of shipping, especially the shipping of oil in the straits," Cohen says.
During the Iran-Iraq war of the 1980s, both Iraq and Iran targeted oil tankers carrying their adversary's oil. Iran now has multiple military platforms it can use to threaten maritime traffic in the Gulf, including hard-to-detect mini-submarines and small, high-speed patrol boats that can swarm a carrier task force.
Its rocket capabilities can be used against ships and strike countries as far away as Israel. Iran has been testing ballistic missiles and probably already has a solid-fuel missile that can reach Israel, according to Jane's Defense Weekly.
U.S. forces in the region are prepared for such attacks but would not be able to prevent them all right away, says Chris Harmer, an analyst at the Institute for the Study of War who worked on military plans for the Persian Gulf as a commander in the Navy. But recent experience shows that Iranian weapons still pose a threat to modern navies.
During Israel's 2006 war with Hezbollah, the Iran-backed militia nearly sunk the Israeli corvair Hanit with an advanced sea-skimming missile that crippled the ship's propulsion system and killed four sailors.
Hezbollah also has thousands of rockets hidden in its base in southern Lebanon that it can launch at Israeli cities. And Syria has missiles it can launch at Saudi Arabia, Jordan, Turkey, Cyprus, and Israel.
Then there is terror and political unrest, a staple of Middle East violence.
Iran, a Shiite Muslim theocracy, could spur uprisings among Shiite populations in Sunni-led Gulf countries, especially Bahrain, which has a Shiite majority, and Saudi Arabia and Kuwait, which have considerable Shiite minorities, Cohen said.
It could also activate Hezbollah and the Iranian Revolutionary Guards Corps throughout the Middle East and the world to hit civilian or military targets in the USA or Europe. It has done so before.
Argentine prosecutors say Iran-backed Hezbollah terrorists were behind the 1992 bombing of the Israeli embassy in Buenos Aires, which killed 29 people, and the 1994 bombing of a Jewish community center in Buenos Aires, which killed 87 people.
Bulgarian prosecutors have tied the 2010 bus bombing that killed six Israeli tourists to a Hezbollah cell, which Israel said was linked to Iran, charges that both Hezbollah and Iran deny.
The United States accuses Hezbollah of the 1983 car bombing of the U.S. embassy in Beirut, which killed 58 Americans and Lebanese. Hezbollah is believed to be responsible for bombings outside the U.S. Marine barracks in Beirut, killing 241 American troops, and a bombing at the French barracks, which killed 58 French soldiers. The group has also carried out a number of kidnappings of Westerners in which they executed hostages or traded them for money or weapons.
But some analysts say Iran's current leadership is unlikely to retaliate.
Suzanne Maloney, an Iran expert at the Brookings Institution's Saban Center for Middle East Policy and a former policy planner at the State Department under George W. Bush, says the public statements of Iran's new president, Hassan Rouhani, show that he "wants to get out of this thing with limited damage to Iran's reputation."
"He clearly understands that if Iran gets into a shooting war with the United States, Europe and international intervention, it's likely to be a quicker path to regime elimination than any other."