(CNN) Two competing alliances are closing in on the Syrian city of Raqqa -- the emblem and administrative powerhouse of ISIS. The Syrian army -- backed by Russian air power -- has made advances from the south-west, while a Kurdish-led alliance supported by U.S. airstrikes is attacking from the north.

Both coalitions want to 'liberate' the city, much as allied and Soviet forces raced for Berlin in 1945. The pressure on ISIS is growing, and not just around Raqqa. Iraqi Kurdish forces are eating away at its territory east of Mosul in northern Iraq; and a variety of Iraqi groups, including the army, have encircled ISIS-held parts of Fallujah, near Baghdad. Defending multiple fronts, ISIS is stretched, with limited ability to reinforce its most vulnerable lines, while conditions for civilians in all these places deteriorate.

But barring any sudden collapse of ISIS, taking Raqqa will be a long slog that may drag on into 2017 -- a slog made longer still by the failure of the many enemies of ISIS to work together.

Why Raqqa matters

It's not the first time in history that Raqqa (a swamp or bog in Arabic) has been 'on the map.' In the late 8th century Haroun al Rashid made Raqqa the capital of his Caliphate, which encompassed much of North Africa as well as Iraq and Syria. Sitting on the banks of the Euphrates, Raqqa was rivalled only by Baghdad in size and became a center of scientific and cultural achievement.

In the endless wars that criss-crossed the region, Raqqa was destroyed by the Mongols in 1265 and withered until becoming an agricultural and industrial center in modern Syria. By the time ISIS arrived in August 2013, it had a population of more than 200,000.

Almost since the outset of the Syrian civil war, Raqqa has been targeted by different rebel groups. ISIS expelled the Free Syrian Army from the city and defeated rival Islamist groups early in 2014.

Raqqa became the most important of the ISIS Syrian holdings -- the first provincial capital it seized -- its administrative center and the city from which terror attacks against Europe are planned.

Holding Raqqa also brought ISIS strategic depth beyond Iraq. It has become a hub for foreign fighters crossing from Turkey, 140 kilometres (100 miles) to the north. Large dams upstream on the Euphrates provide hydro-electric power. Good roads link Raqqa with Mosul and Deir Ezzour, a Syrian city east of Raqqa that is also largely held by ISIS.

The contenders

The United States does not want to see the Syrian regime and its Russian allies win the race for Raqqa.

"It's a difficult thing for people to be liberated from Daesh [ISIS] only to fall under the rule of a regime that has brutalized its own people ," said State Department deputy spokesman Mark Toner last month.

Russian Deputy Foreign Minister Oleg Syromolotov claimed at the end of March that the U.S. and Russia were discussing "specific aspects" of coordination to liberate the city. But U.S. officials deny claims of cooperation in the drive toward Raqqa.

The Syrian army retook Palmyra, some 225km (140 miles) from Raqqa, at the end of March. But it needed a lot of help from Russian helicopter gunships and multiple rocket systems. At least one Russian Special Operations officer was killed, and it took days to dismantle the hundreds of IEDs left behind.

The Syrian army boasted that the capture of Palmyra would be "a launch-pad to expand military operations" into Raqqa and Deir Ezzour provinces. It has made further advances across open country since then but has not taken any notable towns. And army units are so depleted after five years of warfare that they are in no state to launch a frontal assault on the ISIS capital.

The U.S.-backed Syrian Democratic Forces (SDF), an uneasy coalition of Kurdish and Arab groups, are closer -- in some places just 30km (18 miles) from Raqqa's northern outskirts -- and attacking on three fronts.

Their progress across barren windswept plains has been helped by coalition airstrikes and an additional 250 U.S. special forces who are in northern Syria on an "advise and assist" mission. Video has emerged in recent days of them working with Kurdish YPG forces, riding in pick-up trucks mounted with heavy machine guns.

But the Kurdish commander of the SDF offensive (incidentally a woman) says the campaign aimed at liberating the 'north Raqqa countryside' is being slowed by a vast number of IEDs and landmines left by ISIS.

The Turkish question

The Kurds make up the vanguard of the SDF, but while they want to recover Kurdish-inhabited areas and block ISIS access to the Turkish border, they are ambivalent about taking Raqqa itself, a predominantly Arab city.

The Syrian Kurds are instead focused on linking Kurdish 'cantons' along the border with Turkey, creating an autonomous zone that would give them control of much of Syria's border with Turkey -- which is where things get complicated.

The Turks regard the Syrian Kurds as terrorists and have warned the U.S. they will not tolerate further expansion by the YPG [Kurdish fighters] along the border. They are also furious at the U.S. support for the Kurds. President Recep Tayyip Erdogan said at the weekend: "I condemn U.S. support for the YPG. This is not the promise that was given to us."

Turkish opposition to the leading role of the YPG, and its sponsorship of rival groups in northern Syria, may make it more difficult to seal off the 100km (62 miles) of the border that ISIS still holds.

The long war

Despite the progress made in taking territory from ISIS in Syria this year, there's a way to go before its heartland is at risk. Several U.S. officials say there is little prospect of Raqqa falling this year. ISIS has fortified the city's outskirts with tunnels, trenches, minefields and blast walls and it is defended by far more fighters than in Tikrit or Ramadi -- cities that ISIS lost last year.

By some estimates ISIS still has at least 30,000 fighters defending its core territory. But it is under pressure on many fronts simultaneously and cannot reinforce areas under threat.

In Fallujah, near Baghdad, it is fighting a rear-guard action to hold onto neighborhoods it has controlled for more than two years in the face of a coordinated assault by Iraqi forces and militia backed by U.S. airstrikes. Some 50,000 civilians are thought to be trapped in the city.

ISIS has lost of much of western Anbar province in Iraq and a long stretch of the Euphrates River valley in Iraq.

To the east of Mosul a new Kurdish offensive has taken several villages in the past few days.

Even so, ISIS is still capable of offensive action to try to divert or distract its opponents. In the last few days it has launched a surprise attack near the Turkish border, threatening to cut rebel supply lines.

It has also carried out devastating suicide attacks deep inside enemy territory. In the last month, it has carried out multiple suicide attacks in Baghdad.

Seven coordinated suicide attacks against Tartous and Jableh, the heartland of the Syrian regime, killed more than 100 people. These are designed to inflame sectarian hatred and divert their adversaries' resources to defense.

So ISIS is lashing out with a classic terror/insurgent campaign aimed largely at civilians, as well as limited counterpunch offensives. Some analysts e xpect this trend to intensify in June in both Iraq and Syria to mark the month of Ramadan, with the possibility of further terror attacks in Europe.

The 'will to fight'

For the first time, senior ISIS figures are envisaging the next phase of the war. ISIS spokesman Mohammad al Adnani recently acknowledged that ISIS may not in the near future be able to sustain its Caliphate and redefined the meaning of victory and defeat.

Addressing the 'crusaders' in an audio message released on May 21, he asked: "Will we be defeated if you recapture Mosul, Sirte [in Libya] or Raqqa, or all the cities, and for us to return as we were at first? No; defeat is to lose the will to fight."

He also called for further attacks on soft targets in Europe, telling Muslims: "Know that targeting so-called civilians is more beloved to us ... more brutalizing and painful to them and more repulsing."

The group is also restricting the movement of civilians in areas it controls. According to the activist group Raqqa is Being Slaughtered Silently , families have offered as much as $400 to smugglers to help them leave the city. But as in Fallujah, only a trickle have escaped.

The tens of thousands that remain in Raqqa already endure power and water shortages, and rising prices for basic goods, according to activists. They probably face months of hardship and fear under the ever more merciless and paranoid rule of the Islamic State. For the ISIS fighters in Raqqa, much like the die-hard SS units that fought Soviet forces in the rubble of Berlin, neither retreat nor surrender are an option.