The area that is now the Central African Republic has been settled for at least 8,000 years. The people along the eventual border - Sara, Kara, Runga, Arab pastoralists, Kresh, Mbaka, Banda, and many others - did not wake up one morning and discover that their cousins across the riverbank were foreigners. They were traders and intermarriers and herders who followed their cattle where the grass was, which was almost never where a European ministry wanted a border to be. The caravan routes from Tripoli to Bangassou crossed this country for centuries before anyone in Paris cared about it. The rainy season turned the dry bush into marsh, the marshes back into bush, and no one along the Chari noticed when a border moved a thousand meters north because the river had decided to change its mind.

Like most African borders, this one emerged from the Scramble for Africa. The Berlin Conference of 1884 set the rules among the European powers for claiming territory, and France took the lion's share of what is now Central Africa, including the lands explored by Pierre Savorgnan de Brazza - modern Gabon, Congo-Brazzaville - and the upper Niger Valley across Mali and Niger. From these bases French expeditions moved further inland, linking up at Kousséri in northern Cameroon in April 1900. The newly conquered regions were organized into French Equatorial Africa (AEF) and French West Africa (AOF). The border between Ubangi-Shari (the future CAR) and Chad was drawn as an internal administrative line within AEF. When the two colonies became independent in August 1960, the administrative line became an international one. Nothing on the ground changed. Everything in the paperwork did.

Most of the border is defined by rivers, which is elegant in principle and difficult in practice. The Ouham and the Bahr Aouk and the Chari braid and oxbow, laying down new channels in wet seasons, leaving old ones as cutoff lakes. In many places, ironstone plateaus meet clay floodplains, and the water's path depends on how much rain the Congo basin has delivered that year. Where the border uses overland straight lines - three short sections between the Nana Barya and the Petit Sido, a few more connecting stretches - the logic becomes even more arbitrary: a vector drawn across a map is a vector drawn across a map, and the goat herders on either side of it have to learn which village is now which country's business.

The Chadian military has a long history of crossing this border to pursue what it considers security threats. Chad deployed troops to CAR in 2006 at the CAR government's request, after rebels from the UFDR took Birao. Chadian N'Djamena has backed this or that CAR government, or this or that CAR rebel group, as its strategic calculations have shifted. Refugees move the other way. When the War in Darfur spilled across the Chad-Sudan border from 2003 onward, the displacement cascaded south into CAR. When CAR's wars of 2012-2014 and after pushed northward, refugees crossed into southern Chad. The border that was supposed to separate two administrative units has become, in practice, a long line of refugee camps, military crossings, and communities who have relatives on both sides and political allegiances to neither.

Flying over the line, you see very little that looks like a border. The rivers that define most of it are brown threads through green bush in the rainy months, dry courses in the dry months. There are no fences. Even the customs posts are sparse - at Markounda, at Maïtoukoulou, at Goré on the Chadian side, at Bahaï in the far northeast, and a few others. You see the settlements the borderlanders built: Bémal, Bétoko, Nzakoundou, Tissi, Golongoso on the CAR side; Goubeti, Gondey, Tangaray on the Chadian. You see the roads that peter out in rainy seasons and the seasonal wetlands that make entire regions into archipelagos. What you do not see, from the air, is what every family along the border knows: where the old trails are, which ford is safe in August, whose grandfather came from which side.