This coast was Siamese from 1795, and under King Mongkut the settlement was given the name Patchan Khiri Khet, deliberately mirroring Prachuap Khiri Khan on the far side of the Gulf. Then came the machinery of French Indochina. In 1904 France took Koh Kong and Trat in exchange for pulling its troops out of Chanthaburi; three years later a second treaty handed Trat back to Siam, along with the return of Battambang, Sisophon and Siem Reap to French Cambodia - and Koh Kong stayed French. The boundary settled roughly where it sits today, running through the Cardamom Mountains only a couple of kilometres inland of the sea. What the paperwork could not do was move the people. As of 2008 the province counted 48,340 ethnic Thais, and Thai is still spoken across a town whose official language is Khmer.

Koh Kong has around 28,800 residents, and it is laid out as though it expects several times that number. The streets are improbably wide, the blocks are long, and the density is so low that walking between two errands can take a surprising while - most people hire a motorbike, and motorbike taxis run at roughly 1,000 riel a kilometre. Restaurants cluster around the central roundabout; guesthouses line the riverside street and Street 12, which appears on digital maps under the unpromising name Chicken Farm Road. It is not a polished place. Litter is a genuine and visible problem, in the town and along the roads out of it. The Kah Bpow river, which comes down out of the Cardamoms and empties into the Gulf of Thailand here, does most of the work of making the town look like anywhere you would want to linger.

For most of its history the town's connection to the rest of Cambodia was a ferry and a promise. That changed in 2002, when a 1,900-metre bridge went up across the estuary for about US$7.2 million - still the second-longest bridge in the country, behind only the Neak Loeung crossing of the Mekong finished in 2015. The road it carries, Highway 48, is now paved the whole way. The bridge arrived while the region was still shaking off a long war: the Cardamoms were among the final Khmer Rouge strongholds after 1979, and as late as 1998 fighters hit the casino near the border with rocket-propelled grenades, mortars and rifles, wounding a Thai gambler. Air links have been slower. The old strip beside town, VDKK, never carried a scheduled flight and is now an abandoned dirt runway; the province's actual airport is Dara Sakor International, a Chinese-built field on the Botum Sakor peninsula whose 3,200-metre runway has drawn pointed questions from the US Defense Department.

Step off the highway and the reason to be here becomes obvious. Peam Krasop Wildlife Sanctuary, protected by royal decree in 1993, covers 245.91 square kilometres, of which roughly 23,750 hectares is mangrove - a labyrinth of islets and channels recognised under the Ramsar Convention as a wetland of international importance, and one of the largest and most intact mangrove systems in Southeast Asia. Around 700 species have been recorded in it. At Boeng Kayak a kilometre of raised boardwalk runs out over the roots to a suspension bridge and an observation tower, for a little over a dollar. Irrawaddy dolphins work the estuary mouths, and two of Cambodia's three otter species, the smooth-coated and the critically endangered hairy-nosed, hunt these channels. Some 10,000 people depend directly on the sanctuary, which is precisely the tension: since 1979 the province has grown by an average of about 16 per cent a year, and mangrove keeps getting cleared for shrimp ponds.