THE MOOD WAS APPREHENSIVE on the morning of the Upper Tanum water mission, as 77 Filipino marines and seven U.S. Green Berets assembled at the Beach Resort. The Filipinos wore rosary beads and 'do rags with their Vietnam-era jungle fatigues and soup-bowl helmets; many carried ancient M14 rifles or 35-year-old M60 machine guns. Decked out in Kevlar and armed with ergonomically engineered and optically enhanced weaponry, the Americans looked like a lost squad of stormtroopers from Star Wars.

The last two times the marines had ventured into the Tripod, in December 2005 and March 2006, they'd killed approximately ten Abu Sayyaf, but at the cost of two soldiers and another 13 wounded. Many of the marines gathered today were veterans of those battles, though it would be the first visit to the area for the Green Berets. Major Larida addressed the troops. "I would like to tell everyone that the creek we are now going to see—the water is sometimes mixed with blood. Sometimes mixed with blood of Abu Sayyaf, sometimes marine blood," he said, his voice quivering a bit. "So if a firefight starts, everyone down on that creek will be a sitting duck. So today security will have to be to the very highest degree."

Weapons were locked and loaded, bullets clanking into their chambers. The marines pushed out a muted "Oooooooooorah," then piled into the backs of their rusted old personnel carriers and Korean War–era jeeps. The Americans climbed into their armored V-150s.

We lumbered down the coast road, past several jeepneys overflowing with passengers and an old man riding a water buffalo, and through two marine checkpoints to Tanum, a chain of hamlets that stretched two miles up into the foothills. A stronghold of Abu Sayyaf, Tanum had once been home to Ramzi Yousef, and, as recently as five months earlier, a 40-man marine detachment stationed here was harassed nearly every evening by snipers. Now Tanum's leadership, many onetime collaborators themselves, had volunteered to lead the task force into the heart of Abu Sayyaf's regional base camp.

Our convoy halted long enough to pick up ten local guides. The oldest, a fifty-something man wearing a 1963 Vermont Lacrosse Lions T-shirt and carrying a 12-inch blade, climbed into the jeep next to me. Sinewy as a suspension cable, he introduced himself in remarkably good English as the chief of the village of Tanum. His constituents, he said, had petitioned him to ask the marines for a water system after seeing the benefits accruing to other villages cooperating with Sabban's men.

"But why is the Abu Sayyaf letting us do this?" I asked.

"I send one person up there other day to go find Abu Sayyaf," he said. "Tell them the military is helping us with water project. I ask, 'Please no problem?' Then I receive message from them. It said, 'We will go away from your municipal.' "

"But why do they go away?"

"What can they do?"

"Attack us? Destroy the water system?"

"No," he said, with a sly grin. "They know if they destroy water project, the people will hate them."

We turned onto a muddy track, rumbling past a cinder-block mosque strangled by vines and an old schoolhouse pockmarked with shrapnel wounds. The convoy finally ground to a halt about three miles inland as the track funneled into a narrow footpath beneath an archway of coffee and coconut trees. It was humid, the air still. When the engines shut down, I heard parrots squawking.

A marine recon platoon of 26 men went first, led by several Tanum guides. Minutes later, the main body followed, only for Major Larida to halt it about 200 yards in. His radioman had passed him a message: Eighteen Abu Sayyaf had been seen a mile east of the cistern, heading our way.

Larida's fear here was a pantakasi, a Tausug "cockfight derby." Pantakasi occurs when men in a given vicinity with weaponry—pretty much any male over 11 on Jolo—are called out to swarm and annihilate a pinned-down enemy. Eighteen Abu Sayyaf could quickly multiply into a few hundred men firing M16s and swinging swords, as Larida had seen himself the previous November. He summoned one of the Tanum guides, a wild-looking young man in a baseball cap and black hair extensions who I later learned was an undercover Philippine intelligence officer. The marines began taking cover while the head Green Beret, a hulking captain from small-town Massachusetts, ordered his men to spread out. Larida finally made contact with his recon platoon, who'd just secured the hilltop above the cistern. All was quiet, they told him.

There were signs of Abu Sayyaf everywhere: a camouflaged observation post, empty packs of Astro cigarettes and Cloud Nine candy bars (both Abu Sayyaf favorites), a flip-flop, and fresh footprints. "Shit, they were just here," a Green Beret whispered, as he fingered newly cut banana leaves on the observation post.

We had liberated the cistern. There it was, built into a sheer hillside: two cast-iron spigots sticking out of a cement box a little bigger than a VW bus. The guides rushed over. One said something to a marine. He shrugged. Another laughed. A Green Beret guesstimated flow rates, while another eyeballed the gradient of the slope and two more filmed the scene for the intel guys back at the Beach Resort. Despite any historical, religious, or cultural differences, it was pretty clear that the Green Berets, Filipino marines, and Tausug villagers all agreed that this was one damn good water source.

An odd war here, I thought. The allies just ran a major military operation deep into enemy-held jungle and without a shot took a lousy piece of concrete that anywhere else wouldn't even garner graffiti. It wasn't exactly like storming the Normandy beaches, but then again that was the whole point. It wasn't just that the Green Berets and marines were winning the civilians' loyalty; it was that they were forcing the enemy to collaborate in its own defeat.