Ghosts of conflicts past haunt the drive to destroy Isis
Let's be honest, there is justification for bombing Daesh/Isis in Syria. Yet it could still end up badly - like most previous interventions in the Muslim world.
MPs who voted for it could, as Jeremy Corbyn said, find themselves without a hiding place if there are large-scale civilian casualties and it turns into a gory mess, as Iraq did.
What David Cameron needs is a credible plan to bring this organisation down and stop it gaining territory or using its territories as a base to launch attacks on us. That will need ground troops, and the West does not currently have the stomach for this.
Mr Cameron claims there are 70,000 moderate fighters, but Assad's forces, perhaps coupled with the Free Syrian Army, which is led by defecting officers, could prove a more realistic solution.
The key to doing that would be to get an agreement for an interim administration and a timetable for elections once the country was stabilised.
One reason Mr Cameron wants to bomb Syria is to preserve Britain's reputation as a major player. The way that the world views Britain would change if it ignored a call from the UN for every nation to do what it could to defeat Isis, and a direct appeal from its neighbour and ally France to help with retaliatory strikes.
If Britain steps back it will look more like Italy or Belgium; a cultural capital, but with little military reach.
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That is something Mr Cameron will wish to avoid. Certainly, not bombing is unlikely to protect Britain. We saw major attacks foiled in peaceful Belgium, and, last month, Italian police intercepted 800 Turkish-made pump-action shotguns headed for the country.
Good work by the Italians, but do we want to try and foil attacks as they happen or to project power by hitting out when threatened?
One thing the UK can bring to the table is the Brimstone missile. Saudi Arabia is the only other country to have it, and it is using it in Yemen. France, India and the US have also been showing interest in buying the weapon.
Brimstone is the most precise air-launched missile available. It is designed for targeting something like a room, a single vehicle or a small boat. It can also take out tanks, it is armour-piercing and, since it is small, a single Tornado fighter plane can carry several.
The missiles can be targeted at a single vehicle in a convoy, and they will follow the target for miles thanks to a guidance system developed in Afghanistan. The weapon works by following signals planted on targets by special forces or agents. This is a weapon of extra-judicial execution. That doesn't mean that it won't kill civilians, too, though the intention is to minimise such deaths.
The National Security Council has briefed MPs that at least one known suspect in (Isis stronghold) Raqqa is still planning attacks on Britain. The Syrian city was also where Mohammed Emwazi was taken out by a US drone. Emwazi, better known as 'Jihadi John', had beheaded 28 hostages and placed the films online. There have been no more beheadings since he was killed.
Mr Cameron is suffering from the record of Tony Blair and George W Bush. They attacked Iraq over non-existent weapons of mass destruction. The whole thing turned into a quagmire and trust was lost.
There is a better case for war now. Britain is already bombing Daesh in Iraq, and militarily the HQ is in Syria. The Syrian Government has not objected, there is a UN resolution, and there is a clear danger, with recent attacks both threatened and carried out. Allies are also involved, and they have asked for help.
None of that will matter if it turns out the same way as the Iraq war or the Libyan intervention did. If this is to be worthwhile, Britain and the West need a winning strategy that will not leave Syria as a failed state.
Mr Cameron will be judged by results - not rhetoric - at the next election. This is a moment that will define his legacy one way or the other.