Last week, a US drone strike in the Yemeni province of Baydah reportedly killed up to 20 al-Qaeda in the Arabian Peninsula (AQAP) fighters. Among the dead was Shawki Ali Ahmed al Badani, wanted for his role in a plot that led to the closure of US embassies across the region in 2013. While the target was AQAP's leadership, it will have been welcomed by the Iranian-backed Houthi rebels who took Yemen's capital Sanaa in September and have since been in open warfare with AQAP.

In Syria, US air strikes directed against the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front benefit Iran's ally President Bashar al-Assad and their proxy force Hezbollah. Across the border in Iraq, Iranian-backed militias contribute towards President Barack Obama's plan to "degrade and ultimately destroy the terrorist group known as ISIL". While there is no military coordination between the two parties on the ground, it was only recently that Iran's Quds force was assisting Iraqi insurgents kill US soldiers.

Although there is nothing more to this than a mutual interest in the destruction of religious extremists, the fact is there are key strategic forces, including demographics and energy supplies, that are pulling the US and Iran closer together. While this may seem farfetched given the current state of affairs between the countries, Obama's supposed correspondence to Ayatollah Khamenei may presage the start of an irrevocable, if tortuous process, that ultimately delivers a calculated strategic swap of allegiances as Iran replaces Saudi Arabia as the United States' favoured regional ally.

Shelf life



While the US relationship with Saudi Arabia is, of course, strong and of global significance - as the current downward pressures on oil prices show - it does have a shelf life. The current ruler, King Abdullah, is aged 90, and has already outlived two of his chosen successors. The current crown prince, Prince Salman, is 78.

Furthermore, this relationship is primarily based on oil much of which sits in the Shia-dominated Eastern Province. Culturally or ideologically, there is little else that binds the US to the kingdom.

The same can be said of Bahrain where a Sunni elite rules over a disenchanted Shia majority and significant hydrocarbon wealth. The precarious nature of this government was revealed in 2011, when during the Arab Spring, it requested GCC assistance to contain the uprising.

The fact that Bahrain is also home to the US 5th Fleet is another reason for Americans to be flexible in their alignment with long-term regional power shifts.

The final piece of this natural resources jigsaw is Iraq where the US invasion finally settled the result of the Iran-Iraq war with the installation of an Iranian influenced administration in Baghdad governing over the country's vast natural resources much of which lies in the Shia-dominated south.

While there are large differences between the political aspirations of these various countries, when coupled with Iran's own formidable reserves of oil and gas - fourth and second largest in the world respectively - much of the region's natural resources can be seen to be physically located under Shia dominated areas.

The level of change required for Iran to supplant Saudi Arabia as America's pre-eminent regional ally is so great that it is by no means inevitable. Yet such are the strategic forces that draw them together neither is it implausible.

Despite areas of clear economic mutual interest between the US and Iran real rapprochement remains some way off. For many Americans, Iran's funding of Hezbollah justifies its place within the Axis of Evil. For politicians to be in favour of closer dialogue can be contentious, as was demonstrated in the recent midterm election campaigns.

Useful scapegoat

Similarly, in Iran the US is still often referred to as the Great Satan, particularly among its voluble clerics whose message is not one of reconciliation even if their influence is being blunted by increased secularism. The US also remains a useful scapegoat to blame for the country's woes many of which result from corruption and incompetence rather than from economic sanctions.

Added to this antagonism is also the issue of Iran's nuclear ambitions and its relationship with Israel. Yet distant as it might seem, these interdependent issues are reconcilable even if the process is long and tortuous. Time and the cycle of governments and leaders can bring change and allow old ruptures to fade relatively unobtrusively into the past particularly when the pressure of national interest is applied.

The level of change required for Iran to supplant Saudi Arabia as the US' pre-eminent regional ally is so great that it is by no means inevitable. Yet such are the strategic forces that draw them together that neither is it implausible.

In 2002, George W Bush said: "There is a long history of friendship between the American people and the people of Iran. As Iran's people move towards a future defined by greater freedom, greater tolerance, they will have no better friend than the United States of America."

Against a backdrop of rising religious extremism, waning Iranian revolutionary fervour and the imperative of energy security, he might one day be proved right.

Crispian Cuss is a former British Army officer who has worked and lived in the Middle East. He currently acts as a defence and security consultant.