When American military planners launched a ground offensive against the Islamic State in Iraq and Syria in 2016, they knew that the American public was weary of long wars in the Middle East, and that the operation would have to make do with very few Americans troops on the ground. So they relied on a strategy that had not been used much in decades: intensive bombardment by heavy artillery.

Military guidelines said that firing all those high-powered artillery rounds was safe for the gun crews. But an investigation by The New York Times, including interviews with more than 40 gun-crew veterans and their families, found that the troops came home plagued by insomnia, confusion, memory loss, panic attacks, depression and, in some cases, hallucinations, among other symptoms. And because the military thought the blast waves were safe, it repeatedly failed to recognize what was happening to the troops.

Here are five takeaways from the Times investigation.

To defeat ISIS, the United States relied on artillery crews firing more intensively than any had in generations.

The big howitzers used in the height of the offensive against the Islamic State in Syria and Iraq, from 2016 to 2017, could hurl a 100-pound round 15 miles, and gun crews fired them almost nonstop, day and night for weeks on end.

The strategy worked as intended, and the Islamic State was soon smashed to near oblivion. But keeping the number of U.S. troops involved to a minimum meant that each gun crew had to fire thousands of high-explosive shells — far more rounds than any American gun crew had fired at least since the Vietnam War. Some troops fired more than 10,000 rounds in just a few months.

Many members of the gun crews developed devastating and puzzling symptoms.

Each howitzer blast unleashed a shock wave that shot through the bodies of the troops standing near the gun, vibrating bones, punching lungs and hearts, and whipping at cruise-missile speeds through the most delicate organ of all, the brain.