The headline news that American and Iranian officials have discussed their common interest in helping the Iraqi government fight back Sunni militants represents a huge symbolic breakthrough in one of the most dysfunctional relationships in modern times. To gauge the scale of this rare triumph of realism over ideology, it’s worth thinking back to what happened last time the two countries faced a common enemy. Back in 2001 the shared foe was the Taliban. Veteran US diplomat Ryan Crocker has described how after 9/11 American and Iranian diplomats explored ways they could cooperate in ousting the Taliban. Having almost gone to war against their northern neighbours in the 1990s, Iran offered to support US intervention by providing battlefield intelligence and political backing for the Afghan Interim Authority, headed by Hamid Karzai.

Crocker laments how this promising channel was destroyed by the Bush administration’s catastrophic decision to label Iran an ‘evil’ regime in 2002. A year later, the Iranians tried again, reportedly setting out a comprehensive road map for better relations via the Swiss government. The Bush administration, at the peak of its ideological hubris following a devastating demonstration of US hard power in Iraq, not just snubbed their offer but made clear their ambition to change the regime in Tehran once they had dealt with Saddam Hussein. Vice-President Dike Cheney ruled out any notion of engaging Iran with the mantra ‘we don’t talk to evil’.

In the context of recent events, it is ironic that Iran in 2003 reportedly offered to use its influence amongst the Iraqi Shia to help stabilise the country in the chaotic aftermath of the US invasion. We cannot be sure if the offer was workable, but we do know that over the next bloody years the Iranians provided support to Shia political organisations and militias, some of whom lent critical support to Prime Minister Maliki, some of whom killed Coalition troops. Iran’s nefarious influence in Iraq became a prominent feature of government and media commentary.

It is a measure of the seismic changes that have since occurred in Washington, Tehran, and the wider Middle East that Iran and the United States are now broadly on the same side in hoping that these same Shia groups can help repel a Sunni insurgency under the leadership of the barbaric Islamic State in Iraq and Syria (ISIS). Even more startling is the realisation that after 13 years of scathing criticism of Iranian interference in Iraq, the US is to some extent reliant on the 2,000 Iranian troops and militia that have entered Iraq. These troops, including two battalions of the elite Quds Forces that are accused by US officials of assisting Islamic terrorist organisations across the globe, could be critical to the defence of the territory north of Bagdad, including the holy city of Samarra.

This astonishing turn-around in events is matched by the fact that Iran is not protesting the arrival of US troops near its borders. Iran’s new government appears to have accepted that America has a legitimate interest in doing so, in large part because it has come to understand that President Obama is trying to disengage from the region. Reducing America’s footprint in the Persian Gulf has thus become a common aim.

Obama may have sent a carrier to the Persian Gulf and just under 280 ground troops to defend US personnel inside Iraq, but he is being characteristically cautious in deciding whether to direct US firepower against the insurgents. There is not yet decisive political pressure on him to do so and a lot of compelling reasons for him to resist. There is no obvious front line, no major troop-columns to target, and a very real risk that heavy bombing will increase support for ISIS amongst Sunni Iraqis. For these reasons, Obama is waiting to see if the Iraqi army and Shia militias will halt the jihadist’s advance. The White House knows that ISIS has no real capacity either to storm Baghdad or make inroads into the Shia dominated south, including the oil-rich region around Basra.