The case for arming the Syrian rebels rests on the assumption that, once armed with bigger and better weapons, they can topple the Assad regime and that its demise will bring a swifter end to the killing and suffering in Syria. The desire 'to do something' rather than stand by and watch the carnage continue is obviously one motive. Another is to see if the balance can be tipped in favour of some rebels rather than others.
What is not acknowledged, however, is the reluctance of the Western powers (and Western publics) to wade in directly - for fear that they would become embroiled in a situation akin to what followed the invasion of Iraq in 2003. There is no appetite for that and senior military figures in the West have warned their governments that an intervention along the lines of that mounted by NATO in support of the anti-Qadhafi forces in Libya in 2011 is not feasible, because of the nature of the situation on the ground.
Syrian government forces cannot be targeted from the air without endangering civilians, since so much of the fighting is taking place in and around Syria's major cities and towns and the battle lines are fluid. Imposition of a no-fly zone over Syria might stop aerial bombardments of rebel-held areas, but would have to be enforced by Western air forces, which would come under fire from Syrian anti-aircraft batteries and missiles. So even imposing a no-fly zone would mean going to war.
And for such a move there is no legal sanction. Russian and Chinese opposition at the UN is to blame for the absence of a UN resolution permitting use of force in the name of the Responsibility to Protect. The Russian and Chinese stance derives from their refusal to countenance a repeat of the Libyan experience, where, they contend, the NATO powers took advantage of UN condemnation of Qadhafi's implacable and vicious response to his opponents to force the latter's downfall and thence derive advantages for themselves in the new Libya.
In the Syrian case the Americans, British and French bear some responsibility for derailing the possibility, however slight, of an early compromise solution by demanding Assad's exit as a condition for conflict resolution. In doing so they sent the message to the rebels that they were on their side - but then failed to follow through. Assad turned out to be stronger than they anticipated and consolidated his position with the help of Russian and Iranian support.
The United States, Britain and France also discovered that they could not galvanize the Syrian opposition into a cohesive front to whom they could accord the kind of support they had given the Libyan rebels. The Syrian National Council bickered among themselves while the rebel forces on the ground dictated the pace of events. Joined by foreign volunteers or jihadis, the latter were able to make gains, but not enough to 'tip the balance' decisively against Assad.
Now the British and French contend that without assistance the more 'moderate' elements in the Syrian opposition will lose ground to the jihadi extremists whose agenda is both anti-Shia and anti-Western as well as anti-Assad and anti-Iranian. In effect the Westerners want to take sides in a multi-dimensional war, with no guarantee that their chosen protégées will prevail. Most likely, by arming the rebels they will give carte blanche to the Russians and Iranians to reinforce their support for Assad and his allies, including the Lebanese Shia faction Hezballah.
What is contemplated, therefore, is an escalating proxy war in Syria, with not only Russian, Iranian and Arab involvement but also Western engagement. This is not a recipe for 'tipping the balance' against Assad or attending to the humanitarian needs of the millions of displaced Syrian civilians and refugees fleeing across Syria's borders.
As British politicians and diplomats have conceded, had there been an easy or obvious solution they would have taken it by now. Even so, to resort to arming the rebels looks more like a desperate and short-sighted measure than a recipe for ending the conflict. What they might usefully consider is a climb-down - an acknowledgement that this time the Russians are better placed than they to influence events and ask Moscow what they would propose to bring an end to the unfolding catastrophe.