Syria ceasefire: Pentagon disquiet over US-Russia air war plan
It is very early days in the Syrian ceasefire but if the agreement holds, then it could usher in an extraordinary joint air campaign by US and Russian warplanes against so-called Islamic state (IS) and other radical groups linked to al-Qaeda.
Few details of the US-Russia agreement have been given but, at some point, after a week of an effective ceasefire, US and Russian military teams are to meet to begin to chart how they would establish a joint implementation cell to co-ordinate air strikes.
Such a concerted air campaign between the two Cold War rivals is unprecedented. Given the tensions between them over Ukraine, where US and other Western troops are helping to train the Ukrainian military, they are hardly allies.
The question is, do they have enough in common in Syria to make this arrangement work?
In reality it may never happen. The Obama administration is deeply divided over the US-Russia accord.
- What's left after five years of war?
- Syrians enjoy moment of calm
- Life returns to Jarablus after IS flees
- Syrian refugees living in fear as Lebanon tightens its laws
- Behind the scenes at the US-Russian talks in Geneva
- Syria conflict: How will the new truce work?
Secretary of State John Kerry has been determined to go the extra mile to try to bring some measure of respite to the millions caught up in the Syria crisis. Privately, it is said that even he is not optimistic about the deal.
But the Pentagon and US Defence Secretary Ash Carter are deeply sceptical. They have to analyse the practical implications of any concerted air campaign and they do not like what they see.
There are practical, strategic and legal problems to contend with. For a start any combined air campaign will require an exchange of intelligence information about targets. The Pentagon is especially concerned not to reveal any details of its intelligence-gathering capabilities.
The Russian and US air forces have displayed a totally different tactical approach in their parallel air wars over Syria. The US (and other Western air forces) tend to drop almost exclusively precision-guided munitions at point targets, reducing the risk of civilian casualties.
Russian warplanes, in contrast, tend to bomb from higher altitude using "dumb" (that is unguided) bombs. Up till now the US has frequently condemned Russian bombing tactics as indiscriminate. How is it now going to join in a combined air campaign with the self-same Russian air force?
This is especially problematic because as the battle shifts ever closer to an assault on Raqqa - de facto the IS capital - then much of the targeting will increasingly be urban in nature with a growing risk of civilian deaths.
Distinguishing the al Qaeda-linked Nusra Front from other militias (some of which are US-backed) is another problem.
Nusra - it has now re-named itself Jabhat Fatah al-Sham and claims to have broken all ties with al-Qaeda - is one of the more capable militias on the ground and many other groups have formed shifting alliances with it.
They are now supposed to sever all ties and separate their forces. But is this likely to happen? And will the Russians make the same distinctions that US target planners might make?
Question of trust
Then there is the strategic goal of any air campaign. If it is to sweep IS out of Raqqa then who exactly is going to move in on the ground?
This raises all sorts of questions for both Moscow and Washington, with the latter having to worry not least about Turkish-Kurdish rivalries.
But there is a general problem that, as IS control over territory diminishes, many other fighting groups on the ground may be encouraged to take advantage of the new situation and this may place additional burdens on the fragile ceasefire.
Underpinning any joint operation is the question of trust - and there is little of that between the US and the Russians. They have been exchanging some information already so as to "deconflict", as the military term has it, their respective air operations over Syria.
In a nutshell this means that they should avoid coming into contact in the air and that the Russians should not strike any US-linked groups, especially those who have US special forces advisers operating alongside them.