Looking at the Islamic State's string of military conquests in northern Iraq over the past week, one could think the jihadists outnumber their opponents 10 to 1.
After routing government forces in an onslaught launched on June 9, it has managed to hold Iraq's second city Mosul and gone on the offensive again last week, forcing the Kurdish peshmerga to retreat on several fronts and seizing the country's largest dam.
But the jihadist group remains a relatively small force and its strength lies not in numbers. Possible US air strikes, which President Barack Obama said Thursday he had authorised, could stop the rot but IS remains a powerful foe.
Here are five reasons identified by military experts for its military successes:
Newly acquired weapons
The Islamic State has made use of the military equipment it seized from its defeated enemies, including tanks, humvees, missiles and other heavy weaponry.
The amount of hardware, often US-made, the Iraqi army left behind in its spectacular retreat when the jihadists launched their offensive two months ago has transformed IS's capabilities.
"And they keep taking it, they made significant gains of the kind of equipment they needed the most," said Anthony Cordesman, from the Washington-based Center for Strategic and International Studies.
US jets would probably do a better job of destroying that equipment than the Iraqi air force's ageing Sukhois.
IS has long had a foothold in Iraq -- that's even where the group's first incarnation was born in 2004 -- but it became what it is today by fighting in neighbouring Syria.
"Three years of fighting in Syria has provided unparalleled training and learning opportunities for IS," the US-based intelligence consultancy Soufan Group said in a recent brief.
Aggressive tactics by fearless fighters who have amassed huge experience during months fighting the Syrian regime and rival rebels: "That's a kind of fighting people in Iraq weren't used to," said Cordesman.
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IS has picked its battles with great acumen, focusing on Sunni areas where support can be found, key infrastructure or poorly defended sites and by avoiding unnecessary losses to maintain momentum and internal unity.
"They've moved a considerable distance over the past few days but these were very sparsely populated areas and there was very little in the way of defence forces," said John Drake of the AKE Group security company.
"When an opponent is already waning, IS are very good at letting people run away, but against those who really dig their heels in, they haven't cracked many nuts," said Michael Knights of the Washington Institute for Near East Policy.
Retired Iraqi general Ahmed Abdallah Juburi said US air support "could provide some encouragement for Sunni groups and tribes to turn against Daash (Islamic State)" by boosting morale.
IS has used the fear factor to conquer entire towns unopposed. It has posted online grisly pictures of beheadings and mutilated bodies, to recruit radicalised youths but also to scare its opponents.
The jihadists project "an illusion of almost superhuman villainy", said the Soufan Group's Patrick Skinner. Civilians fled the northern town of Sinjar in panic last week when IS warned it was an hour away from entering.
"PR and intimidation is an important tactic for IS," said Drake. "Whether or not they can use all the weapons they seize, they're going to take pictures and use them for propaganda."
Drake argued it was unlikely limited US strikes would "galvanise any jihad. That sort of radicalisation has already occurred."
Iraqi air raids have already caused numerous civilian casualties, and Drake argued that more accurate bombing by the US airforce would be an improvement.
Possibly the single biggest factor making the jihadists look strong is the weakness of its opponents.
"The peshmerga are relatively good by Iraqi standards but they are really light infantry fighters. Those who had experience fighting Saddam Hussein are gone and have been replaced by younger men," said Cordesman, a former US defence official.
"IS has revealed depressing insufficiencies in its opponents, starting with the truly awful performance of the well-equipped Iraqi Army," the Soufan Group brief said.
While training and equipment are needed in the long term, US strikes could temporarily correct that skill imbalance.
Drake said strikes could "soften up some of the IS positions and make it easier for counter-offensives" by the peshmerga. They could also eliminate command centres and harm the group's effectiveness.