(CNN) It's been exactly a year since the Islamic State in Iraq and Syria began its rampage through northern Iraq, seizing Mosul and Tikrit in short order as Iraqi security forces fled south. The group has proved its resilience since, despite thousands of coalition airstrikes and multiple battlefronts across a huge area.

ISIS' most recent successes have come hundreds of miles apart.

Its capture of Ramadi in Iraq and Palmyra in Syria can be explained by its tactics and structure, the weakness or exhaustion of opponents and the support or acquiescence among enough Sunnis in both countries. It may also have benefited, according to some analysts, from cynical power plays in Baghdad.

Even so, taking Ramadi and holding it are two different things. Evidence from previous battles suggests that ISIS doesn't do defense as well as offense, and it is still vastly outnumbered by Iraqi forces. But the longer ISIS fighters are entrenched anywhere, the more difficult they are to expel, and the Iraqi Security Forces clearly aren't capable of the task alone.

In Syria, the seizure of Palmyra gives ISIS access to the main roads to Homs and Damascus and nearby gas fields. It also confirms a shift by ISIS to focus on territory held by the regime of Bashar al-Assad in western and central Syria after a series of defeats at the hands of Kurdish forces supported by coalition airpower in the north.

'Shock and awe'

The term was coined in 2003 to describe the technological power of the U.S. invasion of Iraq. But it can equally be applied to the way ISIS behaves on the battlefield, striking the enemy with massive explosive force.

Back in February, Kurdish commanders near Mosul told CNN how ISIS had sent more than a dozen fuel tankers converted into massive vehicle-borne suicide bombs against their positions. A similar tactic was used to break the resistance of Iraqi security forces in Ramadi.

Michael Knights, an analyst with the Washington Institute for Near East Policy who has spent much time in Iraq, says it's "unsurprising that the ISF in Ramadi finally cracked when struck with a hammer blow -- namely, 28 suicide car bombs in three days, including at least six massive 15-ton armored truck bombs in a single attack."

There were also rumors that thousands of ISIS fighters had come to Ramadi from Syria, likely spread by ISIS' adept use of social media to sow fear.

There is another psychological dimension to ISIS' threat: Enemy soldiers know that they will be killed in cold blood if captured -- probably in gruesome fashion. At Tikrit last June, around Hit earlier this year, in Palmyra in Syria last week, enemy soldiers and other adversaries have been dealt with mercilessly. Summary executions -- en masse -- are part of its mode of warfare. After seizing a Syrian military base near Raqqa last July, it beheaded dozens of Syrian soldiers, posting videos of the barbarity.

According to the Syrian Observatory for Human Rights, it has now begun a similar reign of terror around Palmyra, executing hundreds of captured soldiers and regime sympathizers.

A new style of warfare

Military analysts have been impressed by ISIS' military tactics and flexibility. One senior Kurdish commander told CNN earlier this year that it was a "formidable" enemy that demanded respect. It has commanders with experience and local knowledge who served in Saddam Hussein's military and others who have fought in Chechnya and Afghanistan.

ISIS has perfected the art of "misleading" the enemy, with complex diversionary attacks and mixed signals about its intentions pulling enemies this way and that. As it prepared to attack Ramadi, it also began offensive operations around Baiji and north of Baghdad, deterring or delaying reinforcement of Ramadi by the ISF.

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Few analysts expect ISIS to launch an attack on Baghdad: It knows that its numbers and the capital's vast Shia population would make it a mission impossible . But a devastating campaign of suicide bombings and attacks on the fringes of the city (around Abu Ghraib, for example) would further tie down Iraqi forces.

Turkish analyst Metin Gurcan has noted ISIS' "fluid and decentralized command and control structure; novel hybrid military tactics blending conventional warfare with terrorist tactics, [and] effective use of armored platforms in offensive operations."

It has recently used those tactics in central Syria, beginning with insurgent-style ambushes of government troops, probing weaknesses and killing captives. Then, in mid-May, it launched a more conventional offensive to take territory.

ISIS is also more difficult to target because its units are very small and swift. Gurcan says it frequently deploys "eight to 10 men teams carrying out building-by-building, block-by-block clear and hold operations in urban terrain."

Weak, divided and distrusting

In Iraq and Syria, ISIS is one of the most disciplined players on the battlefield. Its opponents include shifting coalitions of Syrian rebel factions, a Syrian military that is fraying in its fourth year of war and a hollowed-out Iraqi army.

ISIS appears to have calculated that exhaustion and desertion in the Syrian armed forces makes this the ideal time to focus on regime targets rather than a war of attrition with Kurds in the northeast and fratricidal battles against other jihadist groups such as Al Nusra. While there is no indication that the Syrian military is about to be overwhelmed, it is stretched. The regime has moved to crack down on draft-dodging and compensate families of soldiers killed in action. It is also more reliant than before on the Lebanese Shia militia Hezbollah and Iranian support.

The Iraq advantage

In Iraq, units tasked with defending Ramadi had a vast numerical advantage over ISIS. But the evidence suggests the mix of forces -- police, army and tribal militia -- was poorly led, suffering from low morale and in some instances poorly equipped.

"In the 11 months since Mosul fell, only a tiny number of new local forces have been raised in Ramadi -- a weak brigade of 2,000 federal police and a new 1,000-strong unit of tribal paramilitaries," says Michael Knights.

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Sunni tribal fighters had the motivation to defend Ramadi but were not well integrated with other units and lacked the sort of anti-tank weapons needed to deal with ISIS truck bombs. The Sunni tribes have spent a year pleading for more weapons from the federal government -- to little avail, they say, despite repeated promises.

To some observers, the failure to provide weapons to Sunni militia was no accident. Simply put, they say, some Shia politicians were not concerned about Ramadi falling because it posed no threat to the Shia heartland. Nor did they want to see Sunni tribes turned into effective fighting units. Winning back Ramadi would then depend on deploying Shia militia known as Popular Mobilization Units, something Prime Minister Abadi had tried to avoid.

"The shock caused by the fall of Ramadi helped to provide the last push in ending the U.S. and Abadi's resistance to the reflagging of the militias under the banner of Iraq's security forces while keeping [their] structure intact," according to Middle East Briefing.

That is exactly what is happening: the Shia militia is taking the leading role in encircling Ramadi as well as preparing to evict ISIS from the city of Baiji and its nearby refinery farther north.

But ISIS trades on the distrust among Iraqi Sunnis toward the government in Baghdad.

Sunnis between a rock and hard place

The fact that tens of thousands of Sunni civilians have fled Ramadi for an uncertain fate in searing temperatures is a good indication that most are terrified of ISIS. But at the same time, some Sunni tribes are equivocal about fighting for Iraq.

Iraq's Deputy Prime Minister, Saleh al-Mutlaq, who is himself a Sunni, told CNN on Monday: "What is after that? Are they going to live in an area which is going to rebuild again? Is there going to be a reconciliation? Are they going to be included in the government?"

U.S. policymakers see the same dilemma. As one senior U.S. official put it this week: "The rapid integration of the Sunni tribes into the fight alongside other Iraqi forces is essential as they will be the most invested in fighting for their areas."

ISIS can still govern

Despite hundreds of airstrikes on its military infrastructure, ISIS continues to function as a rudimentary government in places such as Mosul and Tal Afar in Iraq, and Raqqa in Syria. It provides security -- on its terms -- and basic services, and has shown itself capable of raising money to finance them. It constantly posts videos of "life as normal" in places such as Mosul: well-ordered streets, markets functioning, hospitals open. The reality is surely less rosy, but a year after falling to ISIS, Mosul still has more than a million residents.

ISIS' control of oil and gas fields in Syria and Iraq has been degraded by airstrikes, but it continues to seize energy sources, most recently the Syrian gas fields of al-Hil and al-Ark, in addition to nearby phosphate mines. It has also developed a lucrative trade in antiquities through the black market.

ISIS has also concentrated its advances on the upper Euphrates and Tigris rivers and now controls huge tracts of both rivers -- critical resources for Iraq and Syria. If ISIS is serious about becoming a self-sustaining state, access to water is vital. So is control over dams and locks as a tool of war. By draining the Euphrates near Ramadi this week, ISIS has opened up new attack routes to the east -- provoking a renewed exodus of civilians.

Exploiting enemies

ISIS is also able to leverage its hold on territory in Syria and Iraq because it can exploit different enemies with different goals, and move fighters and supplies across international borders. In September, it lost the Rabia crossing between Syria and Iraq north of Mosul, but it has compensated by tightening its grip over Anbar and taking the last border post with Iraq still held by the Syrian government at al-Tanf.

Jessica McFale of the Institute for the Study of War writes that even if the Iraqis eventually evict ISIS from Mosul (surely a distant goal), "ISIS will constitute a permanent threat to Mosul if its dominion over the Jazeera desert in western Iraq persists. This outcome is guaranteed while ISIS controls eastern Syria."

But ISIS is vulnerable on defense

ISIS' desire to attack, even when it's defending gains, might yet be its Achilles' heel. It has thrown fighters into futile situations, such as around Kobani in Syria last year and at Eski Mosul in January, even when the odds are heavily stacked against it. But on other occasions -- notably in Tikrit -- it has ultimately retreated to save manpower, using snipers, dozens of IEDs and barriers such as downed bridges, trenches and berms to slow the enemy's advance.

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Michael Knights says ISIS suffers from what he calls "chronic tactical restlessness," an almost pathological need to take the initiative and attack the enemy, even when -- as in Kurdish regions earlier this year -- success was highly unlikely.

Metin Gurcan, writing in Al-Monitor, says that ISIS has so far benefited from the arm's-length coordination of airstrikes and major ground operations. He recommends "close air support that can only be provided by intense cooperation between ground troops and air units." But that would entail risk and commitment that has so far been avoided.

It's also prone to 'overreach'

When ISIS units advanced rapidly through Iraq a year ago, they seized thousands of square kilometers in Kurdish areas they were ultimately unable to hold. Given the number of ISIS fighters and their vast geographical spread in Iraq and Syria, the group is vulnerable to overreach It could have its lines of communications cut and its holdings divided, especially since its ambitious move into central Syria amid the more complex mosaic of factions there.

Even if higher estimates of ISIS fighting strength are accepted -- around 100,000 according to some Iraqi observers -- it is a huge area to hold and govern.

If the real number is much lower, the job seems unsustainable. Without better intelligence on the ground, it's difficult to estimate with confidence how many fighters ISIS has and their different motives for fighting, how many have been killed and how quickly they are being replaced.

U.S. officials say some 13,000 ISIS fighters have been killed by airstrikes since August out of a force of some 30,000. Other observers say it's implausible the group could have lost nearly half of its strength and still be so effective across such a huge area.

ISIS now has a new challenge to factor in. Other jihadist groups in Syria, such as Jabhat al Nusra and Ansar al Islam, have essentially declared war on it in northern Aleppo. Ansar al Islam, in a statement issued this week, said that if there was no "decisive and real position to deter their aggression, then it is the beginning of the end."

ISIS' leadership is still determined to expand its caliphate in Iraq and Syria, seeing it as a step toward an even greater regional presence. It frequently talks of "remaining and expanding" in its publications. But it is fighting from the suburbs of Baghdad to the fringes of Damascus, on Syria's borders with Turkey and Lebanon. According to the Syrian Observatory for Human Rights, ISIS now controls some 95,000 square kilometers in Syria.

Location, location ...

And its credibility rests with holding cities. A Caliphate that includes Mosul, Raqqa, Ramadi and Palmyra demands attention in a way that a Caliphate limited to the Anbar desert does not. And holding and administering cities, unless their inhabitants leave or are expelled, takes a lot of resources.

But the same challenge confronts its opponents. Even if Ramadi can be retaken -- and there's little progress to report so far -- persuading the displaced to return home and providing governance and services is a huge challenge, one readily acknowledged by President Barack Obama's special envoy, Gen. John Allen.

ISIS has a very deliberate policy of wrecking the places from which it retreats. But if this challenge is not met, ISIS would soon be back.