Iran's influence in the Middle East has been heightened by the US "war on terror", according to a report.
Iran is important to other Middle East countries, the report says
The Chatham House think-tank study said the US had eliminated regimes in Iraq and Afghanistan but failed to replace them with stable political structures.
Recent conflicts involving Israel in Lebanon and Gaza have added to instability, the report continued.
Iran has now superseded the US as the most influential power in the Middle East, it concluded.
The report - called Iran, Its Neighbours And Regional Crises - by the London-based think-tank said: "There is little doubt that Iran has been the chief beneficiary of the war on terror in the Middle East.
"The United States, with coalition support, has eliminated two of Iran's regional rival governments - the Taliban in Afghanistan in November 2001 and Saddam Hussein's regime in Iraq in April 2003 - but has failed to replace either with coherent and stable political structures."
The report said Iran was too important for political, economic, cultural, religious and military reasons not to be taken seriously by other countries in the Middle East or the rest of Asia.
"The US-driven agenda for confronting Iran is severely compromised by the confident ease with which Iran sits in its region," the study added.
Iran's importance in the region helps explain why it is able to resist outside pressure over its nuclear intentions, the report continued.
But Edward Luttwak, a US Pentagon advisor, told the BBC that although Iran had gained influence, it was also dealing its own domestic problems.
He cited calls for autonomy by the Azeris, the largest ethnic minority in Iran, an insurgency by the Kurds and said that growing secularism meant some believed the government was out of tune with its people.
He said: "There are insurgencies all around the fringes and in the core of the country, in Tehran, half of the population won't talk to them."
One of the authors of the Chatham House report, Dr Ali Ansari of the University of St Andrews, told BBC Radio Five Live: "We've seen really since 9/11 that the chief beneficiary of America's global war on terror in the Middle East has been the very country that it considers to be a major part or a founding member of the axis of evil.
"And that basically tells us that there's an enormous incoherence in American approach to the Middle East.
"They simply haven't managed to work out a strategy and a policy that will work and will achieve results."
Chatham House is a non-profit, non-government organisation for the analysis of international issues.