In October 1967, the American poet Robert Lowell, who was jailed for his pacifism during the Second World War, marched on the Pentagon in Washington as part of an anti-Vietnam war demonstration. Before a crowd of as many as 100,000 people and alongside Norman Mailer, he read the final stanza from one of his finest poems of public address, "Waking Early Sunday Morning":

Pity the planet, all joy gone
from this sweet volcanic cone;
peace to our children when they fall
in small war on the heels of small
war - until the end of time
to police the earth, a ghost
orbiting forever lost
in our monotonous sublime.

Lowell understood the burden, loneliness and corruption of America's chosen role as world policeman. He had a premonition of what the future held for a nation that was, because of its imperial ambition and sense of its own higher moral purpose, destined repeatedly to be ensnared in conflicts in distant lands.

This week, once again, people - conscripts, mercenaries, civilians - are falling in some small war, this time in Libya, and Britain, America's hawkish, ever-diligent and willing accomplice, is at the forefront, cheered on by our war-whooping press. The Labour Party has dutifully fallen in behind David Cameron's coalition government, as if this were a moment of heightened national emergency. It is not.

Competing objectives

All three main parties are espousing the doctrine of liberal interventionism on humanitarian grounds. The murderous actions of Colonel Gaddafi, until recently a despot with whom Britain could do business, must be stopped, and the rebels bolstered and protected even though we know very little about who it is we are meant to be supporting and what the duration and limits of our support should be.

For too long, the debate about liberal interventionism has been conducted in absolutist terms. On the one hand, you have the neoconservatives, or ultra-interventionists, with their Manichaean world-view, and their beliefs that rogue states can be bombed into submission and democracy imposed through violence and external agencies, rather than through internal pressures of the kind which led to the fall of tyrants in Egypt and Tunisia. On the other hand, you have those who would oppose all intervention as a form of neo-imperialism.

The art of successful foreign policy is the art of measuring competing objectives; of knowing when to intervene (as in Kosovo in 1999 and Sierra Leone in 2000) and when not to (Iraq in 2003). Under Tony Blair, liberal interventionism itself became a kind of absolutist dogma. For Blair, there was a "moral obligation" to intervene "to make the world better". Emboldened by the success of his action in Sierra Leone to defeat the militias that had laid waste to an entire nation, and the Nato-led assault on Serbia during the Kosovo war, Blair wrongfully supported the Americans in their illegal war in Iraq. The inconsistencies of his positions abounded. Why intervene in Iraq and not in, say, Iran or Darfur? Similarly, why now should we intervene in Libya and not in Yemen, where civilians are being murdered by an autocrat every bit as repugnant as Gaddafi?

Perhaps we should turn to the great liberal philosopher J S Mill for help. In 1859, writing against the backdrop of the Crimean war, the Indian mutiny and the construction of the Suez Canal, Mill published an essay titled "A Few Words on Non-intervention", still one of the best I have read on the subject. Mill was not opposed to all foreign adventurism. As a servant of imperialism, he believed in the "civilising" mission of the British empire, but he set limits on when a state should intervene in the internal affairs of another, especially during a civil war or revolt. Mill was conscious that any foreign intervention would be viewed from the outside as an act not of humanitarianism, but of cynical self-interest. It's all about the oil! He believed that if a people did not have "a sufficient love of liberty to be able to wrest it from merely domestic oppressors, the liberty which is bestowed on them by other hands rather than their own, will have nothing real, nothing permanent".

How robust is the rebellion in Libya? How do the aspirations of the rebels differ from those of the socially networked young protesters who brought down Hosni Mubarak in Egypt? How committed are they to liberty, for whom and of what kind, or is this merely a secessionist conflict between rival tribes?

Self-determination

Mill continued: "When the contest is only with native rulers, and with such native strength as those rulers can enlist in their defence, the answer I should give to the question of the legitimacy of intervention is, as a general rule, No."

No people, Mill thought, "ever was or remained free, but because it was determined to be so . . . If a people - especially one whose freedom has not yet become prescriptive - does not value it sufficiently to fight for it, and maintain it against any force which can be mustered within the country, even by those who have the command of the public revenue, it is only a question of how few years or months that people will be enslaved."

Unlike in Sierra Leone, where militias were supported by an outside agent - the Liberian warlord Charles Taylor - or Kosovo, where ethnic Albanians were struggling against the aggression of a Greater Serbia, the struggle of the Libyan rebels is against a native ruler. How long would they have been prepared to continue the freedom fight? We shall never know, because of the haste with which the western powers have rushed to intervene as they seek to police the earth, ghosts orbiting forever lost.