American forces made one of their most effective hits against the Islamic State on Nov. 15, when U.S. planes destroyed 116 tanker trucks used by the terrorist organization to transport the stolen oil that is its financial lifeblood.

American A-10 and C-130 warplanes targeted a group of about 300 trucks near Abu Kamal, in Syria. Given that the Islamic State is thought to have just over 1,000 trucks in its entire fleet, the group of 300 represented a huge target for U.S. planes.

At a Pentagon news conference last Wednesday, reporters wanted to know why American forces did not take out more than 116 trucks. Why not all 300, or something close to that? A U.S. official said the American attackers simply ran out of ammunition.

"There were 300, I think, to begin with, and then you hit 116. Why didn't you go back?" a reporter asked Operation Inherent Resolve spokesman Col. Steve Warren.

"Frankly, the aircraft expended 24 500-pound bombs, and all of their ammunition," Warren answered. "So they — they shot everything they had and then they had to go home."

Journalists had another question: If oil is vitally important to the Islamic State, why didn't the U.S. hit the tanker trucks long ago, given that the American anti-ISIS operation began in September 2014?

"If it's so important to cut off the oil shipments, the critical revenue source for ISIS, why did it take so long to take out 116 oil tanker trucks?" a reporter asked.

Warren explained that American officials were deeply worried about harming the truck drivers, who were working for the Islamic State but might not be ISIS themselves. U.S. officials settled on a plan to drop leaflets on the trucks about 45 minutes before the raid, warning the drivers that an attack was coming, while U.S. pilots flew low passes over the area. Planning all that took time.

"This is our first strike against tanker trucks," Warren said. "We have been striking oil infrastructure targets since the very beginning of this operation. What we found out was that many of our strikes were only minimally effective. We would strike pieces of the oil infrastructure that were easily repaired."

Warren said U.S. officials conducted a study and decided to focus on the Islamic State's oil distribution network — trucks. But the problem was the drivers.

"We have not struck these trucks before," Warren said. "We assessed that these trucks, while although they are being used for operations that support ISIL, the truck drivers, themselves, [are] probably not members of ISIL; they're probably just civilians. So we had to figure out a way around that. We're not in this business to kill civilians, we're in this business to stop ISIL — to defeat ISIL."

American officials decided on the leaflet-and-buzz plan. "We spent some time developing ... the leaflets, the low pass," Warren said. "We did some ... some strafe runs as well — to kind of shoo people away without harming them. So we had to go through that whole process of one, determining whether or not we felt it was in our best interest to strike these trucks. And then once we determined that, yes, it is in our interest to strike these trucks, how do we go about ensuring that we're able to mitigate the potential of civilian casualties? And these things take time."

Finally, when all the work had been done, 300 Islamic State trucks were found in one place. It was an inviting target.

"These trucks were queued up," Warren said. "They were sitting sort of on flat desert. There wasn't really anything recognizable as a road around. It was just out there on the desert floor in the vicinity of one of these oil fields, one of these oil facilities that we've been striking. So they were queued up there waiting to take on their illicit oil. ... So these trucks were — they were just sitting there, not moving."

It was an extraordinary opportunity to take out somewhere between one-quarter and one-third of the Islamic State's entire truck fleet. And after destroying 116, the Americans ran out of ammunition. The remaining 184 trucks were not destroyed.

Beyond the out-of-ammo problem, reporters raised the issue of the U.S. forces' extreme care in avoiding civilian casualties. If American forces won't hit any target if there is any fear that any non-ISIS person might be harmed, might that not prolong the time it takes to destroy the Islamic State, which is killing civilians right and left?

"Is there a Catch-22 about not striking civilians and avoiding civilian casualties when, over the past year, the $50 million in oil revenue generated has been going to killing a large number of civilians?" a reporter asked. "Is there any risk that avoiding civilian casualties is actually prolonging more civilian casualties?"

That was a very thoughtful question, Warren conceded, and an issue American war planners think about constantly. But Warren didn't have an answer. "It's something that we wrestle with every day," he said.