ONE reason American officials give for Barack Obama’s reluctant decision to arm Syria’s rebels is that it will bolster and unify the ranks of the more moderate fighters among those battling President Bashar Assad. The administration says it plans to funnel arms through Selim Idriss, a defected general who heads the political opposition’s Supreme Military Council. Yet choosing who gets what and trying to meld the fractious bunch into a coherent body to receive the weaponry will be difficult. For one thing, 27 months into the war, the rebels have so many supply lines that few outsiders know precisely who is at the receiving end (see map).

The main conduit runs from Turkey into northern Syria, where the rebels control swathes of land, including all the main border crossings. Large shipments of arms come mainly from Qatar, which flies in weapons from Libya, an ally that was plentifully supplied by the Qataris during the uprising two years ago against Muammar Qaddafi. Turkish intelligence agents accompany the weapons into liberated parts of Syria, to be handed over to chosen groups, such as Liwa al-Tawheed, a large one active around Aleppo. The arms are distributed down the chain.

Rebel commanders say Turkey’s fairly open-door policy also lets arms provided by private donors be smuggled in. Some come from Libyan revolutionaries, others from rich Salafist sheikhs in the Gulf. A black market functions energetically, too. That may explain some of the sophisticated anti-tank missiles seen in rebel hands. The New York Times has reported that Qatar has supplied some man-portable air defence systems (MANPADS), despite American pleas that they should not.

The American government has used the northern route from Turkey to send in food, telephones and other non-lethal supplies. Yet, once it begins to send the lethal stuff, it is more likely to funnel most of it through the rebels’ second main supply route, into southern Syria from Jordan.

Last year the Saudis, worried by what they saw as the Qataris’ reckless taste for extreme Islamist rebels, moved their base there from Turkey. With CIA help, the Saudis have sent batches of arms to fighters in the south who have yet to make as much headway as those in the north. Such weapons, including a large shipment bought in Croatia at the end of last year, have gone through the rebels’ military council in the southern province of Deraa, to be sent on to umbrella groups such as Liwa al-Islam.

Jordan keeps as tight a control over its border as it can, because it is petrified of blowback by Islamist fighters who might threaten its own regime. Rebel commanders say arms supplies going into southern Syria from Jordan are minor compared with those from the north.

It is unclear how much suppliers in the north and south co-ordinate with each other. Hugh Griffiths of the Stockholm International Peace Research Institute, a think-tank, notes a recent increase in Jordanian aircraft flying from Jordan to Turkey. The Saudis still send some arms in from the north.

Smaller channels also operate elsewhere. Lebanon’s former head of intelligence, Wissam Hassan, who was assassinated in Beirut in October, probably by Syrian agents, is believed to have sent at least one shipment of arms to rebels via Lebanon, but such routes are generally limited to smugglers rather than state providers. Rebels say that supplies from Iraq are also limited, though an activist from Deir ez-Zor, Syria’s biggest city near the border with Iraq, says that some supplies coming in that way are destined for rebel jihadists.

For the Americans, arming the rebels from the south may be most practical, as the rebel-controlled area in southern Syria is smaller, there are fewer jihadists there, and Jordanian and Israeli intelligence provide good maps of the area. America already has 700 troops in Jordan, along with Patriot missiles and F-16 fighter jets.

But different routes give different results. Arming rebels in the north would help achieve some of America’s stated political aims. Rebels control a large area there, allowing leaders such as Colonel Abdul Jabbar Akaidi, the head of Aleppo’s military council, to operate inside the country. The Syrian National Coalition, the opposition’s main political umbrella, has suggested setting up an administration in that area, if it could be better protected from attacks by forces of the regime.

Some say America should work from both sides, as well as send more sophisticated weapons, if Mr Obama’s decision to arm the rebels is to have a real effect. Much will also depend on the degree to which Americans can directly oversee the flow of supplies. During the 1980s, when America armed the mujahideen in Afghanistan, only about 50 CIA agents monitored the supply of weapons funded by Saudis and delivered by Pakistani security people. It will be equally hard, in Syria, to ensure that supplies do not seep through to groups such as Jabhat al-Nusra that are linked to al-Qaeda.