BEIRUT, Lebanon — One week after Islamic State fighters attacked a prison in northeastern Syria, where they have held out despite a heavy assault by a Kurdish-led militia backed by the United States, the terrorist organization published its version of what had gone down.

In its official magazine, it mocked how many times in its history its foes had declared the Islamic State to be defeated. Its surprise attack on the prison, it crowed, had made its enemies “shout in frustration: ‘They have returned again!’”

That description was not entirely wrong.

The battle for the prison, in the city of Hasaka, killed hundreds of people, drew in U.S. troops and offered a stark reminder that three years after the collapse of the Islamic State’s so-called caliphate, the group’s ability to sow chaotic violence persists, experts said. On Saturday, about 60 ISIS fighters still controlled part of the prison.

In Iraq, ISIS recently killed 10 soldiers and an officer at an army post and beheaded a police officer on camera. In Syria, it has assassinated scores of local leaders, and it extorts businesses to finance its operations. In Afghanistan, the withdrawal of American forces in August has left it to battle the Taliban, with often disastrous consequences for the civilians caught in the middle.

The Islamic State, which once controlled territory the size of Britain that spanned the Syria-Iraq border, is not as powerful as it once was, but experts say it could be biding its time until conditions in the unstable countries where it thrives provide it with new chances to expand.

“There is no U.S. endgame in either Syria or Iraq, and the prison is just one example of this failure to work toward a long-term solution,” said Craig Whiteside, an associate professor at the U.S. Naval War College who studies the group. “It really is just a matter of time for ISIS before another opportunity presents itself. All they have to do is to hang on until then.”