The battle to retake the Iraqi city of Mosul from ISIL involves a bewildering array of forces some of which would normally be sworn emenies. There are about 5,000 American advisers and trainers with the Iraqi army, in de facto alliance with the Iranian-allied Shia militia leaders. The Turkish army has a small base at Bashiqa near Mosul, in open defiance of the Baghdad government, and is working with Kurdish forces, who are more frequently the army’s enemies than allies. As the Iraqi armed forces tighten their grip on Mosul, a traditionally Sunni city, some of its units are flying banners of Shia martyrs.

This is an explosive mixture. But the contradictions can probably be smoothed over for as long as it takes to recapture the city after two years under the rule of the jihadists. Certainly the United States has exerted huge diplomatic and military efforts to keep the uneasy coalition together, arousing suspicions that Washington is looking for a victory by the date of the presidential election, November 8.

What happens afterwards is anybody’s guess. But it is clear that the Mosul campaign has shone a harsh spotlight on Turkey’s increasingly assertive military stance in its southern neighbours. Turkish troops are in place in both Syria and Iraq and are not going to leave.

This has been a gradual process but it is now clear that Turkey sees an enduring military and political role in Iraq, a development underpinned by neo-Ottoman rhetoric from its president, Recep Tayyip Erdogan.

There are several reasons for Turkey’s forward-leaning stance – domestic politics, religion and regional ambitions.

Ankara now sees both its neighbours as “weak or failed states” which will produce instability for the foreseeable future, according to the Turkish-American analyst, Soner Cagaptay.

The result is a permanent military presence to create a cordon sanitaire. This is an attempt to seal off Turkey’s own restive Kurdish minority from their ethnic kin in Syria, where allies of the separatist PKK are dominant, and to prevent the PKK’s spread into northern Iraq where they already have a foothold.

Mr Erdogan has said that Turkey is not happy with the borders it was forced to accept in 1923. At the time, it wanted to extend southward to include Mosul. This raises the question of whether it will annex territory to redress its losses in the colonial era. But that is unlikely – to challenge official borders would open the way for dreams of a pan-Kurdish state claiming south-eastern Turkey.

Of greater long-term concern is Mr Erdogan’s expansive rhetoric. On October 22 he described Iraq, Syria and Bosnia as “part of our soul”. Such words seem to presage an interventionist foreign policy in the former lands of the Ottoman Empire, the predecessor of the Turkish Republic.

The government in Baghdad is concerned that once established in Iraq, Turkey will not just intervene to fight the rise of the PKK and its allies but also to act as the protector of the Sunnis there. This could lead to war, the Iraqi prime minister, Hayder Al Abadi, has warned.

It is not just a question of supporting Turkey’s co-religionists. There are clear signs that Mr Erdogan is keen to pursue a more energetic regional policy. In short – to challenge the seemingly unstoppable rise of Iranian influence.

Such goals have a solid underpinning in Ottoman history. The Ottomans and Iran’s Safavid Empire fought a century of wars for control of modern-day Iraq, with the Ottomans finally triumphing in 1639. The Treaty of Zuhab was a major defeat for the Safavids, taking Iraq out of the Persian sphere and establishing it as a “citadel of Arabism”.

For Iran the defeat turned out to be a long-term blessing: it established Iran within borders which have lasted to this day and enabled the empire to transition smoothly into a nation state.

The downsizing of the empire of the Turks was more recent and drastic. Its modern borders were drawn after the First World War. The new Turkish Republic for–swore regional entanglements and looked westward to Europe for three generations. But so long as the Arab states are weak, the old empires cannot sit tight.

Iran’s Islamic Revolution of 1979 unleashed new energies and spread the mullahs’ influence to Syria and Lebanon and then, thanks to the American invasion of 2003, to Iraq. The Americans destroyed the Saddam Hussein dictatorship, which entrenched Sunni dominance under the guise of secularism, and replaced it with a confession-based democracy and put Shia parties in power. Many of those parties’ leaders were closely linked to Iran.

Signs of a similar imperial revival can be seen in Turkey: the religious energies unleashed by Mr Erdogan have spawned a mood of Ottoman nostalgia that has coalesced into a desire to roll back Iranian influence.

How far Mr Erdogan can turn his desires into reality is moot. His first foreign power play – ­removing Syrian president Bashar Al Assad – has failed, and only served to redouble the regime’s support from Iran and Russia. Russia is unlikely to look kindly on Turkey, a Nato member, extending its influence into a region where it believes it has greater interests than America.

There is also a question mark on Turkey’s economic ability to support its ambitions. The same could be said of Iran, which has been haemorrhaging money in support of the Syrian regime and may have lost 700 combatants.

For the moment, the US has abandoned major military interventions into the Middle East, except for supporting the fight to destroy ISIL, which it sees as a threat to the homeland.

With America taking a back seat, there is a struggle for leadership. But it would have taken a very insightful analyst to predict that the withdrawal of one imperial power would revive a contest between two defeated empires over the legacy of wars fought four centuries ago.

Alan Philps is a commentator on global affairs

On Twitter @aphilps