The use of car bombs in guerrilla warfare is not a new tactic, but the Islamic State (IS) has deployed an unprecedented number of them as it has fielded a more conventional force in order to capture cities in Iraq and Syria.

In contrast to their use as traps or along roadsides, IS has used vehicles rigged with bombs and improvised explosive devices (IEDs) much as the US has traditionally used air power: as a means of assaulting ground forces and opening up targets to attacks by infantry units.

"It is a very effective weapon and we have taken great efforts to advance our targeting capabilities in order to allow us to find these locations where IEDs are being produced and target them appropriately," Marine Corps Brig. Gen. Thomas Weidley, the chief of staff for the air war in Iraq and Syria, told reporters recently.

The asymmetric nature of the tactic can seem astounding — with the Pentagon spending more than $9 million a day to fight the insurgency, IS is constructing bombs virtually for free.

IS is manufacturing the weapons at an alarming rate, using everything from fertilizer to military grade explosives. The wide stretches of territory controlled by the group — which is also known as ISIS, ISIL, and by its Arabic acronym Daesh — make it easier to hide the factories that assemble the bombs, and the fact that many of the areas they have taken over are rural means they likely have a supply of fertilizer that will not run out anytime soon. the New York Times reported in May that IS was continuing to import fertilizer from Turkey as well.

During the takeover of the western Iraqi city of Ramadi, IS fighters used car bombs on a regular basis for two months to slowly push back the defense of pro-government forces.

"Sometimes it is a hundred meters in a day, sometimes it is just 50," a police officer in Ramadi told VICE News during a visit to the city in March.

Other officials, speaking on a day when IS had detonated at least three car bombs in an early morning battle, said that such events had become commonplace. Pro-government forces said that as IS launched its final push to take Ramadi, it deployed as many as 30 car bombs, some strong enough to level city blocks. The attack also included an armored bulldozer that removed defenses before IS blew it up as well.

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"I believe that the Tamil Tigers were the first to integrate vehicle bombs with an infantry assault," said Mike Davis, the author of Buda's Wagon: A Brief History of the Car Bomb. "Although most of their bombings emulated the 1983 suicide bombings of the US and French barracks — that is to say, solo attacks on military bases or cities — their elite kamikazes, the Black Tigers, used powerful car or truck bombs to initiate several battles, as have the Taliban and more recently IS. But the Ramadi attack was shock and awe on a wholly different scale."

Though there is no official public count, hundreds of car bombs were used against Iraqi government targets and US troops between 2003 and 2011, when the IS predecessor al Qaeda in Iraq was at the forefront of the country's insurgency. The Department of Defense is presently weighing whether to make the statistics it has collected public.

The US has delivered anti-tank rockets to the Iraqi government to help mitigate the car bomb threat.

"This is a tactical issue, but we have accelerated our delivery of anti-tank rockets to make sure that the Iraqis in the field have the ability to combat what is now ISIL's weapon of choice, just these massive suicide truck bombs," said Brett McGurk, the Deputy Special Presidential Envoy for the Global Coalition to Counter ISIL, earlier this month.

The munitions are "useful to counter IS car bombs in the front lines, but using such rockets is impossible in the cities," said Jamil Abdul Qadir, an Iraqi military analyst in Baghdad. "Talking about defeating IS' car bomb capability is very premature."

Davis is also skeptical of the efficacy of such countermeasures.

"You can't defend against them," he said. "At the end of the day, all you can do to prevent or counter car bombs is to ban traffic. You simply cannot have checkpoints and walls and barriers everywhere."

Yet that is an accurate description of much of Baghdad has looked over the last 12 years. Entire neighborhoods were surrounded by concrete blast walls and given single entry and exit points to prevent such attacks, which contributed to a tendency US soldiers' tendency to open fire on cars that failed to come to a stop ahead of checkpoints between 2003 and 2011. Such measures were designed to protect US troops, but often resulted in civilian deaths.

It was often impossible to get a civilian car anywhere within a block or two of an American base without passing through checkpoints. The International Zone or "Green Zone" that houses the Iraqi parliament and other government buildings, as well as the US Embassy, is almost as heavily fortified as it was prior to the 2011 withdrawal of US troops.

Other countermeasures have been taken, though some have been laughable. In 2008, the Iraqi government purchased "bomb detectors" that were actually novelty golf ball locators. Bizarrely, the fake bomb detectors remain in use at checkpoints all over Baghdad, even after the British man who sold them to the Iraqi government was convicted of fraud in the UK. He is now serving a 10-year prison sentence.

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The attacks have had a predictably unsettling psychological effect on Iraqi troops. The final onslaught in Ramadi, in which as many as 30 car bombs were deployed, caused many of the pro-government forces in the city to simply flee.

"They knew that the driver is a suicide terrorist," said an Iraqi soldier standing guard in Khadimiya, a neighborhood in west Baghdad that is home to a historic mosque has been the site of repeated attacks. "Seeing a car bomb heading towards me is a nightmare. I would vanish immediately."

The soldier said he had witnessed two such attacks in the last year.

"We have been ordered by our commander to do our best to kill the driver before he detonates his car," he added. "But how can we guess that it is a car bomb?"

IS is not the only one to improvise in the current fighting — the Syrian government has been widely criticized for the use of "barrel bombs," crude IEDs that are often made from oil drums and packed tightly with nails and metal scrap that are simply dropped from helicopters flying high above their targets to avoid anti-aircraft fire. The unguided weapons frequently kill civilians.