If the Obama Administration follows through with its planned “shot across the bow” of Bashar al-Assad’s Syrian regime, it should be prepared to deal with the unintended consequences of such action. Some of the most troubling repercussions could come from Iran, Assad’s chief ally.

Iran’s Supreme Leader, Ayatollah Ali Khamenei, yesterday warned, “We believe that the Americans are committing a folly and mistake in Syria and will, accordingly, take the blow and definitely suffer.”

Iran’s Revolutionary Guards reportedly have already made preparations to launch attacks against the U.S. The Wall Street Journal today reported that U.S. intelligence officials had intercepted an order from a high-ranking Iranian Revolutionary Guard general to pro-Iranian Iraqi Shiite militants calling for attacks on the U.S. embassy in Baghdad and other targets in the event of an American strike against Syria. The Iranian order came from General Qasem Soleimani, the head of the Quds Force, an elite special forces unit within the Revolutionary Guards that coordinates Iranian support for foreign groups.

The Quds Force orchestrated attacks by Iraqi Shiite militias against U.S. forces in Iraq. It was also involved in the foiled plot to assassinate the Saudi Ambassador in Washington in 2011 and a broadening terrorist campaign against Israel and the U.S. in recent years.

Tehran has invested heavily in the Assad regime and is shoring up its ability to inflict continued carnage on the Syrian people. It provides money, arms, intelligence support, military advisers, and “volunteers” from the Revolutionary Guards, including many from the Quds Force. It has also deployed militia fighters from Hezbollah, its Lebanese surrogate, and from radical Shia militias in Iraq to fight inside Syria against Assad’s opposition.

If the U.S. attacks Syria, Iran is also likely to threaten to close the Strait of Hormuz, as it has threatened many times in the past. The narrow waterway is a strategic chokepoint at the eastern mouth of the Persian Gulf through which flows about 20 percent of the world’s oil supplies, or about one-third of the world’s seaborne oil shipments.

In 2007, The Heritage Foundation conducted a computer simulation and gaming exercise to determine the economic and policy ramifications if Iran were to forcibly shut down the strait. Based on 2007 data, the study concluded that if the strait was blockaded for one week, oil prices in the U.S. would almost double to $150 per barrel, up to 1 million jobs could be lost, and the nation’s gross domestic product could fall by $160 billion within one year.

Iran would be unlikely to follow through on its threats to close the strait, because doing so would jeopardize its own oil exports, cut off most of its imports, and risk a military clash with the U.S. that could result in the destruction of its nuclear weapons program. But Tehran has periodically ordered gunboats from the Revolutionary Guards naval force to harass shipping and U.S. naval vessels passing through the strait. Washington should be prepared to deal with that possibility and the worst-case scenario in which Iran attempts to close the strait in a future crisis.

The troubling bottom line is that the U.S. risks substantial blowback from the use of force in Syria, and the longer the attack is put on hold, the more time Iran has to plot a response.