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If there is one regional player that gained the most from America's gamble in Iraq, it is Iran. With its invasion in 2003, the United States ousted Tehran's sworn enemy, Saddam Hussein, from power. Then Washington helped install a Shiite government for the first time in Iraq's modern history. As U.S. troops became mired in fighting an insurgency and containing a civil war, Iran extended its influence over all of Iraq's major Shiite factions.

Today, the Iranian regime is moving to exert influence beyond its Iraqi proxies, and is comfortable taking overt military action. There is no one to restrain Tehran, and the rise of ISIS, which views Shiites as apostates, threatens the interests of all Iraqi Shiite factions and of the Iranian regime.

In late November, Iranian warplanes launched several airstrikes against targets in eastern Iraq, pushing back ISIS militants who had neared a self-declared "buffer zone" that Iran established along the border.

It was the latest example of how Iran is expanding its military and political influence in Iraq, a country wracked by a complex civil war that leaves it open to outside manipulation.

Iraq is at the center of several regional proxy battles: Iran is heavily involved in shaping Iraqi policy, while ISIS represents spillover from the Syrian civil war next door. The militant group is also a byproduct of the Gulf Arab states that support Sunni jihadists in both Syria and Iraq. And the United States is bombing ISIS and other jihadist groups in Iraq and Syria.

The Iranian regime has several concerns: Iraq provides strategic depth and a buffer against Saudi Arabia and other Sunni Arab states that are competing with Iran for dominance over the Persian Gulf. More broadly, Tehran wants to ensure that Iraq never again poses an existential threat to Iranian interests, as Saddam did when he invaded Iran in 1980, instigating the eight-year Iran-Iraq war that devastated both countries.

Saddam was supported by the Sunni Arab states and most Western powers. (The Shiites are the majority in Iraq, but since its independence in 1932, the country had been ruled by the Sunni minority until the U.S. invasion in 2003.) Iran will do whatever is necessary to keep a friendly, Shiite-led government in power in Baghdad.

The long game

Iran has excelled at playing the long game, especially in Iraq.

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