Add this to the dictum about choosing your friends wisely: Be careful which allies you arm in your fight against a mutual enemy. A new European Union–financed study of the weapons Islamic State of Iraq and Syria (ISIS) militants are using in battle has found that more than 80 percent of the ammunition originated in the U.S., China, and Russia. The organization conducting the study, Conflict Armament Research, examined 1,730 recovered cartridges made as far back as 1945.

The information is collected on the website iTrace, which requires registration. The New York Times dug into the data, however, and has some numbers. The ammunition came from 21 countries, but about 19 percent originated in the U.S. — very likely captured from Iraqi security forces — but another 8.5 percent could have been sent into the region by the U.S. as well. Another 26 percent came from China, a large supplier of military armaments worldwide, and an equally large number of cartridges were made in the Soviet Union in the 1970s and 1980s, probably taken from Syria's Russian-supplied army.

Syrian officers have probably sold some small number of armaments to ISIS, rebel commander Fouad al-Ghuraibi tells The New York Times. But ISIS also picks its battles carefully, and with an eye toward capturing weaponry. "When battling against the Syrian Army, ISIS chooses to fight in a specific battle on a specific front only when the investment is appealing," he said: "There will be warehouses to capture." Peter Weber