David Cameron's case for military intervention in Syria is like that old Groucho Marx joke: “We have to have a war. I've already paid a month's rent on the battlefield”.
Cameron wants us urgently to sign up to his war effort.
But on the basis of what?
He claimed this morning that he had "a credible military strategy" to defeat Islamic State.
The more he spoke, the more apparent it became that he has nothing of the kind.
Worse, he has learned nothing from the mistakes that led us into Iraq more than a decade ago.
Syria exists as a united entity only on the maps.
On the ground, it is fractured between competing interests.
Syria's Russia-backed secular dictator, Bashar Assad, controls a significantly shrunken territory.
The Kurds, who are bravely fighting Islamic State, dream of self-rule in the north.
Then there is ISIS itself, and an ensemble of overlapping opposition groupings whose ideological leanings are virtually impossible for us to determine.
Cameron wants to enter this mix.
He wants to bomb ISIS from the skies and hopes to use "moderate" rebels as ground forces.
Simultaneously, he wants to rid Syria of Assad and install a representative government that can finish off his fight against ISIS.
This is an ambitious plan. It is also fantastic.
First, Cameron's claim that there are 70,000 "moderate" rebels is extremely dubious.
The source of this claim is the joint intelligence committee - the same source which warned us that Saddam Hussein could bomb British bases in 45 minutes.
Western governments have been struggling to vet the identities of refugees who have been flooding Europe over the last several months.
Can we seriously believe that it is possible for us to vet the ideological complexion of 70,000 rebels in a war zone?
What was the methodology used to arrive at this number?
Did intelligence officials collate this figure from the estimates given to them by commanders of local rebel units?
If so, how reliable are those commanders and what was the criteria used to assess the fighters?
We could benefit from the experience of the United States, which spent more than half a million dollars training rebel forces – only to lose control of them later.
As General Lloyd Austin told a Congressional hearing earlier this year, out of the hundreds of rebels it trained, the US could track down only “four or five” fighters.
The rest either bolted or defected to other factions.
Cameron offends the intelligence of his colleagues, and the British public, when he bandies this figure: "70,000 moderate rebels" is the new 45 minutes .
Second, Cameron let loose his belief that "the best ground troops" against ISIS "should be the Syrian army".
This demonstrates a worrying ignorance of Syrian realities on Cameron's part.
For nearly half a decade, western experts have been predicting Assad's demise.
Yet he remains in power.
Tens of thousands of Syrian Arab Army troops have laid down their lives in his defence.
It is absurd for Cameron to believe that they will switch their loyalty to a Western-backed government once Assad is deposed from power.
When I reported from Syria in 2012, non-Western diplomats were openly scornful of what they saw as Western naivety - even after the experience of Iraq - about Syria.
As the ambassador of a major Asian country told me, "the French, British and Americans have no understanding of what's happening here".
Many decent people are justly appalled by Assad.
But outrage is not policy.
There exists no force in Syria capable of reconstituting the country if Assad goes.
And as we have seen in Libya, actions driven by good intentions can produce deadly results.
Cameron has no workable plan.
Instead, he is attempting what the French call on s'engage, puis on voit : first engage, then figure out.
But there is no reason to believe that Syria's miseries will be relieved - or Europe's security enhanced - if Britain started bombing a country that's already being pounded daily by multiple powers.
It would be inexcusable to enable Cameron to ignore history and plunge Britain head first into this homicidal cauldron.
Kapil Komireddi has reported from South Asia, Eastern Europe, and the Middle East