Key point: In Iraq and Afghanistan, the United States found that conquering a country is the easy part. It’s the occupation that proves costly.

Since assuming office in 2009, President Barack Obama has consistently held that the United States would carry out airstrikes to prevent Iran from acquiring a nuclear weapon. This position is supported by the vast majority of U.S. policy makers, lawmakers and the political elite, regardless of political affiliation.

Nonetheless, it is also generally agreed that airstrikes against Iran’s nuclear facilities would only have a limited impact on preventing Iran from acquiring the bomb. To be sure, a concerted airstrike effort against Iran would delay its ability to build a nuclear arsenal by several years. Nonetheless, Iran would be able to rebuild its nuclear facilities before long, especially given the windfall in economic relief it would undoubtedly receive once the sanctions regime against it unraveled in response to America’s military action.

The only military action that can truly prevent Iran from acquiring a nuclear weapon, then, is for the United States to invade and occupy the country, potentially turning it over to a U.S.-friendly regime that would uphold Iran’s non-nuclear status. Despite the widespread support in the United States for preventing Iran from building a nuclear weapon, this option is almost never proposed by any serious observer.

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Part of this undoubtedly reflects America’s fatigue following the wars in Iraq and Afghanistan. However, it goes much deeper than that—namely, while Iran’s military is greatly inferior to the U.S. armed forces, the U.S. military would not be able to conquer Iran swiftly and cheaply like it did in Iraq and Afghanistan. In fact, Tehran would be able to impose prohibitive costs against the U.S. military, even before the difficult occupation began.

Iran’s ability to defend itself against a U.S. invasion begins with its formidable geography. As Stratfor, a private intelligence firm, has explained, “Iran is a fortress. Surrounded on three sides by mountains and on the fourth by the ocean, with a wasteland at its center, Iran is extremely difficult to conquer.”

While the “stopping power of water” has always made land invasions far more preferable for the invading party, the age of precision-guided munitions has made amphibious invasions particularly challenging. As such, the United States would strongly prefer to invade Iran through one of its land borders, just as it did when it invading Iraq in 2003.

Unfortunately, there are few options in this regard. On first glance, commencing an invasion from western Afghanistan would seem the most plausible route, given that the U.S. military already has troops stationed in that country. Alas, that would not be much of an option at all.

To begin with, from a logistical standpoint, building up a large invasion force in western Afghanistan would be a nightmare, especially now that America’s relationship with Russia has deteriorated so greatly.

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More importantly, however, is the geography of the border region. First, there are some fairly small mountain ranges along the border region. More formidable, going from the Afghan border to most of Iran’s major cities would require traversing two large desert regions: Dasht-e Lut and Dasht-e Kavir.

Dasht-e Kavir is particularly fearsome, as its kavirs are similar to quicksand. As Stratfor notes, “The Dasht-e Kavir consists of a layer of salt covering thick mud, and it is easy to break through the salt layer and drown in the mud. It is one of the most miserable places on earth.” This would severely constrain America’s ability to use any mechanized and possibly motorized infantry in mounting the invasion.

Iran’s western borders are not any more inviting. While northwestern Iran borders Turkey, a NATO ally of the United States, Ankara refused the United States permission to use its territory for the invasion of Iraq. Regardless, the Zagros Mountains that define Iran’s borders with Turkey, and most of Iraq, would make a large invasion through this route extremely difficult.

The one exception on Iran’s western borders is in the very south, where the Tigris and Euphrates rivers collide to form the Shatt al-Arab waterway. This was the invasion route Saddam Hussein used in the 1980s. Unfortunately, as Saddam discovered, this territory is swampy and easy to defend. Furthermore, not long after crossing into Iranian territory, any invading force would run into the Zagros Mountains. Still, this area has long been a vulnerability of Iran’s, which is one of the reasons why Tehran has put so much effort into dominating Shia Iraq and the Iraqi government. Unfortunately for any U.S. president looking to invade Iran, Tehran has largely succeeded in this effort, closing it off as a potential base from which America could attack Iran.

Thus, the United States would have to invade Iran from its southern coastline, which stretches roughly 800 miles and is divided between waterfront adjoining the Persian Gulf and Gulf of Oman. Iran has been preparing for just such a contingency for the better part of a quarter of a century. Specifically, it has focused on acquiring the capabilities to execute an antiaccess/area denial strategy against the United States, utilizing a vast number of precision-guided and nonsmart missiles, swarm boats, drones, submarines and mines.

As always, Iran benefits in any A2/AD campaign from the geography of the Iranian coastline; in The Revenge of Geography, Robert Kaplan observed of Iran’s coastline, “its bays, inlets, coves, and islands [make] excellent places for hiding suicide, tanker-ramming speed-boats.” He might have added hiding ground-launched missile systems.

Michael Connell, director of the Iranian Studies Program at CNA, further reflected: “Geography is a key element in Iranian naval planning. The Gulf’s confined space, which is less than 100 nautical miles wide in many places, limits the maneuverability of large surface assets, such as aircraft carriers. But it plays to the strengths of Iran’s naval forces, especially the IRGCN. The Gulf’s northern coast is dotted with rocky coves ideally suited for terrain masking and small boat operations. The Iranians have also fortified numerous islands in the Gulf that sit astride major shipping lanes.”

All of this plays into an Iranian A2/AD strategy. Back in 2012, the Center for Strategic and Budgetary Assessments (CSBA) studied how Iran would use A2/AD against the United States, stating:

“Iran… is developing an asymmetric strategy to counter U.S. operations in the Persian Gulf. This strategy may blend irregular tactics and improvised weapons with technologically advanced capabilities to deny or limit the U.S. military’s access to close-in bases and restrict its freedom of maneuver through the Strait of Hormuz. Iran’s ‘hybrid’ A2/AD strategy could exploit the geographic and political features of the Persian Gulf region to reduce the effectiveness of U.S. military operations. Such an approach may not, in itself, be a war-winning strategy for Iran. Significantly raising the costs or extending the timelines of a U.S. military intervention may, however, create a window of opportunity for Iran to conduct acts of aggression or coercion.”

As this implies, the United States would sustain significant damage and casualties trying to establish a beachhead in southern Iran. America’s challenges would not end with establishing this beachhead, however, as it would still have to conquer the rest of Iran.

Once again, geography would work to Iran’s advantage, as almost all of Iran’s major cities are located in the north of the country, and reaching them would be a herculean challenge under the best of circumstances. For starters, the terrain—as always—would be challenging to transverse with a large invading force. More importantly, Iran is enormous. As Stratfor notes, “Iran is the 17th largest country in world. It measures 1,684,000 square kilometers. That means that its territory is larger than the combined territories of France, Germany, the Netherlands, Belgium, Spain and Portugal—Western Europe.”

Of course, U.S. forces would not be operating under the best of circumstances. In fact, Iran’s Revolutionary Guards Corps (IRGC) has long planned on mounting an insurgent and guerrilla campaign against an invading force trying to reach Iran’s northern cities from its coastlines. Referred to by the IRGC as a “mosaic defense,” the plan would incorporate the joint efforts of the IRGC, Basij and regular armed forces. Connell describes it as follows:

The mosaic defense plan allows Iran to take advantage of its strategic depth and formidable geography to mount an insurgency against invading forces…. As enemy supply lines stretched into Iran’s interior, they would be vulnerable to interdiction by special stay-behind cells, which the IRGC has formed to harass enemy rear operations.

The Artesh, a mix of armored, infantry and mechanized units, would constitute Iran’s initial line of defense against invading forces. IRGC troops would support this effort, but they would also form the core of popular resistance, the bulk of which would be supplied by the Basij, the IRGC’s paramilitary volunteer force. The IRGC has developed a wartime mobilization plan for the Basij, called the Mo’in Plan, according to which Basij personnel would augment regular IRGC units in an invasion scenario.

IRGC and Basij exercises have featured simulated ambushes on enemy armored columns and helicopters. Much of this training has been conducted in an urban environment, suggesting that Iran intends to lure enemy forces into cities where they would be deprived of mobility and close air support. Iran has emphasized passive defense measures—techniques used to enhance the battlefield survivability —including camouflage, concealment and deception.