This is probably a naive view, but I believe fighting both President Assad and ISIS -- as recommended by Obama and Cameron -- is NOT a strategy.
I'm not sure that's necessarily representing Cameron's position. His is even more confused.
Cameron's line is a qualified fight against ISIS in Iraq. But he ends up assisting ISIS in Syria, by prioritising the fight against Assad
It's frankly a bonkers position, and not surprisingly, is finally falling apart. He's playing a fight - help -fight formation
I think it's also worth revisiting the word "fighting". Let's not over-state things. Flying around at 20,000 ft and programming co-ordinates into a computer isn't necessarily 'fighting' in the sense that its a game changer. It was revealed last month that since they started bombing in Iraq about 3 years ago, the RAF estimate they've killed about 300. To be honest, ISIL replenish that within a week. It's what's led the Commons Foreign Affairs Select Committee to conclude that we aren't doing anything substantive. The sub text is that the whole campaign is being based around the needs of one individual, and to allow him to ponce around the world stage pretending he's relevant. With Russia involved now, the select committee are quite rightly saying that the risks don't justify the paltry reward any longer
So ultimately someone has to go in on the ground, and when they do you can see that any operation will inevitably get bogged down in rubble filled urban landscapes which are notoriously difficult to assault with all the concealed sniper positions they present defenders with. ISIL will have dug tunnels and made use of cellars too (as a heavy gas chlorine works quite well on those! - but we'll see if this ever becomes an issue if they ever get to Raaqa)
The big flaw in Cameron's position of course is that he was actively trying to degrade the best engaged fighting proxy available to the west. It was madness! Sheer bloody madness
We're now learning from the American's (in a desperate attempt to re-write their misjudgements) that they knew all along that the Free Syrian Army were unlikely to simultaneously defeat ISIL, Assad, and Al Nusra, but that they needed to prove their mettle to get a place at the negotiating table - what!!!
Ideally you could do with the FSA going over to the government, but that isn't going to happen. Indeed, the FSA's second preference vote seems to be more aligned towards Al Nusra, as this has tended to be the pattern of defections to date (which tells its own story perhaps)
About the only thing Clive has said that makes sense, is that a quick defeat doesn't look likely. The time where that might have been delivered has passed, something which is largely down to western and gulf state meddling in the name of enemy identification (I wouldn't exonorate Turkey either).
We need to try and seal up the borders and stop the supply routes now (easier said than done) but so far the Islamic State have seemingly not had too much difficulty brining in new fighters and getting their hands on weapons and ammunition. They don't have any ports, or any operational air supply. The tracks across deserts are slow and hazardous as well, and could be patrolled by high altitude drones that can stay on station for 36 hours. They should be one of the easier lumps of territory to isolate