It's very hard to choose an excerpt from this. You should try to read it all:
Goodbye Iraq, Hello Iran -- by Dore Gold
.... Clearly, without the presence of U.S. forces in Iraq, the Iranians will be able to infiltrate Iraq again and influence its internal stability. On December 7, the deputy head of operations for U.S. forces in Iraq, Lt.-Gen. Frank Helmick, admitted that there are still "security gaps" in the new Iraqi Army that Washington is leaving behind. This raises questions about whether Baghdad has the capabilities to face the Iranian challenge without U.S. help.
Iran has also been developing its relationships with Shiite religious institutions in Iraq, where the Shiite holy cities of Najaf, where the tomb of Ali, the first Shiite Imam, was built and Karbala, where the tomb of his son, Hussein, who was the second Imam is located. In Samarra, there is the burial place of the tenth and eleventh imams; it is also where the twelfth imam disappeared, according to Shiite tradition, until he returned as the Mahdi. Tens of thousands of Iranian pilgrims come to these holy sites every month. Like with the Iraqi political parties, Iran is making payments to Shiite leaders and institutions, although at present, the highest Iraqi Shiite leader, Ayatollah Sistani, opposed Iranian policy in Iraq in the past.
In short, Iran has strong strategic, economic, and religious interests that it will pursue in Iraq after the US withdrawal. Its main goal is to make sure that Iraq never again becomes a strong power, like in the era of Saddam Hussein, which can threaten Iranian security. To achieve this aim, Tehran will try to reduce Iraq to a satellite state that will support Iran's regional strategy in the Middle East. Already, Iran is demanding that al-Maliki back the embattled regime of Bashar al-Assad in Syria, rather than the Syrian opposition which Turkey is helping. Al-Maliki is backing the Iranian position.
The U.S. withdrawal from Iraq is producing a strategic change in Israel's situation in the Middle East. From 1980 until 1988, Iranian expansionism was blocked by the Iran-Iraq War, with the exception of Lebanon, which Tehran exploited as a front against Israel and the West. From 1991 until the end of 2011, Iraq served as a strategic barrier to Iran, and since 2003, that barrier was reinforced by U.S. military power. Now it appears that the Iraqi barrier is being removed as it increasingly comes under Iranian influence....