IF A TREE FALLS The loneliest militant hippie.

THERE'S A WORD to describe someone who willfully burns down a building, or several—sometimes for the rush, but usually for some kind of gain, either financial or political or ideological.

It used to be "arsonist." No longer.

These days, if someone torches a building to send a message, they might wind up branded something way more sinister: terrorist. No one dead? No one hurt? No one cares.

That dubious shift in American jurisprudence—and whether it rings true in a world of suicide bombers and planes slicing through skyscrapers—is the central question raised by If a Tree Falls: A Story of the Earth Liberation Front, a surprisingly even-toned documentary that charts the brief, brilliant flash of one of the organization's lonely cells.

The story is told primarily through the eyes of one particularly hapless activist, Daniel McGowan. We meet him under house arrest in New York City, a few years after his heady prime, when he helped activists in Eugene strike at symbols of the logging industry, from a tree farm (in this case, mistakenly) to corporate offices to a university lab.

McGowan's fat. He bickers with his sister. He gets married. He's hardly the type to hole up in a bunker and issue fatwas. And, yet, once he finally pleads guilty, he winds up heading to a prison specially built to hold such men.

If a Tree Falls is careful not to be too sympathetic to McGowan and his friends. True, we see the devastating vistas of the clear-cut mountaintops that've stoked the ELF's rage, but later, we hear cops and lawyers remind us of the chance for collateral damage. The targets of the fires speak of the fear they had to endure.

To its credit, If a Tree Falls wades into one other powerful question: What part do the authorities play in breeding radicalism? When peaceful activists are ruthlessly beaten and pepper-sprayed, why not fight fire with fire?