The decision to impose a no-fly zone over Libya is unfortunately the easy part. The idea that Libya is the model of how liberal interventionism ought to be done, in contrast to the "disaster" of Iraq, is flawed. David Cameron rightly made much of the differences between the two cases. This time, the cheese-eating surrender monkeys are at the leading edge of the formation. This time, the Arab League, a regional alliance of Muslim countries, is also at the forefront, while America is reluctant, merely supportive in the background. And the United Nations has passed a resolution explicitly authorising the use of military force.
All these are good things, and certainly throw up some piquant paradoxes. Two weeks ago Mr Cameron was being mocked for the failure of his no-fly zone proposal to fly. The typical black humour in military circles was that he had been misunderstood and was actually describing the current state of the Royal Air Force. Ed Miliband, who had "welcomed" the plan when the Prime Minister suggested it on 28 February, asked with ambiguous innocence if Cameron could "clarify where that proposal now stands" two days later. Well, it turned out that Cameron did know what he was doing after all.
The critics of interventions past have been, metaphorically, to Damascus. Sir Malcolm Rifkind, the appeaser of Serbian aggression in the Balkans in the 1990s, last week urged the Egyptians to send a brigade into Benghazi. The Liberal Democrats, who fought the 2005 election on a retrospectively anti-war platform are now in a government authorising their first new military action.
The decision by the UN Security Council has certainly flushed out some of the weaker arguments against intervention. Jeremy Corbyn, the Labour MP who is opposed to everything, pointed out last week that "abuses of human rights and the oppression of civilians are not unique to Libya", and asked: "Is the Prime Minister now suggesting that we should develop a foreign policy that would be prepared to countenance intervention in other countries where there are attacks on civilians, such as Saudi Arabia, Yemen, Oman or Bahrain?" This is what my friend Sadie Smith calls the Why Should I Tidy My Bedroom When The World's Such A Mess theory of foreign policy. There was a lot of it about in 2003 as well.
At least it is not as bad as the ghastly Stop the War Coalition, incorporating the once noble if possibly mistaken cause of the Campaign for Nuclear Disarmament, both of which long ago fell into the hands of tiny Marxist splinterlets. Last week they organised a demo outside Downing Street to protest against the UN vote to impose a no-fly zone. The Stop the War Coalition has now been exposed as the opposite of its name, just as CND seems credulously indifferent to nuclear armament by dangerous regimes.
Putting such distractions to one side, then, the crisis in Libya has brought the debate on liberal interventionism back to earth after eight years orbiting in the vacuum left behind by the Iraq war. The choices are difficult and marginal. They are often 60-40 decisions rather than the absolute certainties sometimes declaimed by opponents of the Iraq invasion. That is why the clarity of the no-fly zone decision is more apparent than real.
We were here in the run-up to the invasion of Iraq too. Because of what happened later, many people have forgotten how it looked at the end of 2002. The civilian population of Iraq had some protection from no-fly zones in the north and south of the country. The French, at that stage, indicated that they would support an invasion and offered 15,000 troops. Many Arab and Muslim countries supported UN resolution 1441, passed unanimously by the Security Council, including Syria, which gave Iraq a "final opportunity" to comply or to face "serious consequences".
Cameron claims that resolution 1973, passed with five abstentions (Russia, China, Germany, Brazil and India) last week, was more explicit in its authorisation of the use of military force. But it is limited to declaring a no-fly zone, leaves open the possibility of arming the anti-Gaddafi forces, and very clearly rules out ground forces from outside.
It also avoids the phrase "all necessary means", which is the accepted formula for authorising military force (although it has been used only twice, against North Korea in 1950 and Iraq in 1990), referring to "all necessary measures" instead.
The hard choices on Libya are yet to come, therefore. It is to be hoped that the unity of the region and the world against Gaddafi will weaken him, and that arms by sea and air support for the rebels will be enough to topple him. But it seems unlikely. And then what?
That is when it will become impossible to sustain the unity of the UN and the fragile consensus in Britain. So far the coalition has been robust and the Labour Party has offered its full support. I am told that Deputy Prime Minister Nick Clegg regards last week's show of international unity as "a full stop after Iraq", but it could turn out to be more of a comma.
If it turns out, as is surely likely, that the people of Libya can be protected – as demanded by the UN resolution – only by intervention on the ground, then it is going to be hard to follow through the brave words of the joint statement issued by the "UK, US and France, with the support of the Arab States", which was the order in which Downing Street listed the actors. President Barack Obama was clear on Friday that the US would not deploy ground troops – mind you, Bill Clinton said the same thing about Kosovo, and it was his change of mind that eventually forced the issue.
Then the difficult decisions will have to be faced. First, there will be, "What's it got to do with us?" Cameron tried to answer that last week, sounding an uncanny echo of Tony Blair on Kosovo and Iraq – the dangers of an emboldened rogue regime on the southern border of Europe. That just about holds while it is a bombing campaign, but a stiffer test has to be passed for ground troops, assuming the British Army, still engaged in Afghanistan, has any to deploy.
Unless Gaddafi and his regime falls soon, it is hard to see any way in which this is going to end comfortably for Cameron and Clegg, with all the doubters over Iraq happily united behind a new, consensual doctrine of liberal intervention.