Johann Hari: What if the anti-slavery campaigners had listened to the carping cynics?

Some will claim the campaign was a waste of time. I can hear the jeer now: "Is poverty history yet?"

Drink lots of water, take a few paracetamol, and get ready for a thumping hangover. After the beautiful sight of Edinburgh surrounded by a giant white band made up of a quarter of a million protesters, after the sweet songs of Live8, whatever is announced from Gleneagles this week will be a disappointment. The three life-saving aims of Make Poverty History - trade justice, drop the debt, more and better aid - will not be achieved in a week, and a cackling chorus of cynics will pounce on this to claim the campaign has been a waste of time. I can hear the jeer now: "Is poverty history yet?"

When you hear the acid of scepticism being poured over everything that has just happened, respond with a date: 22 May 1787. On that day, 12 men gathered in a printing shop in east London and began a campaign that seemed insanely naive: the abolition of slavery. The campaigners were sitting at the heart of an empire built on forced labour. Almost all the world's crops were grown by slaves who earned no money, worked 14 hours a day and died (at best) in their forties. The abolitionists' goal seemed as distant as the idea of a world without absolute poverty seems now.

But as Adam Hochschild explains in his brilliant new history of the movement, Bury the Chains: Prophets and Rebels in the Fight to Free an Empire's Slaves, within little more than a single lifetime, slavery was abolished in the British Empire. Yet there were whole decades when - like today - their cause seemed unachievable. But they did not surrender to those who said that blacks were forever condemned to servitude because of their inherently corrupt and stupid natures. They did not listen when Adam Smith declared that "slavery ... has hardly any possibility of being abolished. It has been universal in the beginnings of society", or when Edmund Burke said that the opponents of slavery had "decent" ideas but were "naive beyond measure".

Make Poverty History is the anti-slavery campaign of our time. If this sounds like hyperbole, think about the facts. How will it seem to future generations that, just a few hours' flight from London Heathrow, a child died every three seconds from the entirely preventable diseases of poverty? And how will they judge the fact that while this happened, the rich world actually made Africa's problems worse? We are not simply talking about the failure to offer a helping hand; too often, we are talking about punching Africa in the face.

The list of blows is long. Today, rich countries force Africans to wrench open their economies, and then flood them with European and American subsidised agricultural products. The result? Farmers in Ghana get to market and find European chickens and American grain at a fraction of the price it really costs to grow and sell them. This is how Africa's attempts to provide for itself are smothered in their cots.

Bad enough - but the list continues. Today, the rich world physically prevents many poor countries from manufacturing cheap generic Aids drugs, simply in order to protect the profits of massive corporations. Today, the rich world is demanding that African people repay the debts of long-gone dictators, at the expense of building schools and hospitals. Today, the rich world is knowingly causing the planet to slowly, silently warm, already provoking crop failure and looming Darfur-style resource wars across Africa.

The report on this one-sided boxing match - Mike Tyson versus Dot Cotton - could continue for a long-time. Will all these crimes really look in the end so different to the crimes of our great-great-grandfathers? Yes, there are dictators and corruption in some parts of Africa (although certainly not all). But to use this as an excuse to carry on doing our own terrible harm to the continent is like beating up an old woman and then explaining that she already had cancer so it's not your fault she's dead.

The anti-slavery movement showed - for the first time in human history - that people are capable of tirelessly fighting for somebody else's rights. The best banner I saw in Edinburgh said simply, "Imagine it was your child." Never underestimate the revolutionary power of empathy: it can make people campaign against their own interests on behalf of people half a world away who they will never meet. The population of Sheffield in the 1780s made their wealth primarily by manufacturing scythes, knives and razors, most of which were sold to slave traders. Yet they signed a mass petition in 1789 saying "[we] consider the consider the case of the nations of Africa as [our] own." The anti-slavery movement ultimately cut British GDP by 1.8 per cent for more than four decades - but morality trumped individual economic interest. This will have to happen again - with the rich world's farmers and corporations especially - if Africa is to succeed.

But the anti-slavery campaigns also showed that it is essential to build the broadest coalition possible. In the next few weeks, I fear the Make Poverty History coalition will begin to fray. Bob Geldof and Bono will almost certainly welcome the inches of progress made by this G8, and the more radical wing of the movement will turn on them. I understand their concern: at what point does praising the leaders of the rich world for small steps in the right direction actually turn into a whitewash for the people at the head of the system doing so much harm to Africa?

Yet the anti-slavery movement succeeded because it had people who worked both within established power structures urging gradual change, and outside the system fighting for more revolutionary solutions. William Wilberforce was an ultra-conservative who opposed the extension of the vote beyond a tiny aristocratic elite. He holidayed with absentee plantation owners and acted on the assumption that even the ugliest slave-herders simply did not understand what they were doing. Geldof and Bono are the Wilberforces of Make Poverty History, appealing to men like Bush and Berlusconi on the assumption they simply don't understand the harm their system is doing.

It might be stomach-churning, but it is worthwhile provided it happens in parallel with men like Thomas Clarkson, who fought as part of a more radical movement. If Make Poverty History is going to fly, it needs both wings.

Two hundred years ago, a generation of people decided to end a cancer that had always existed: slavery. They did not wait for benign leaders that never came, nor did they let one disappointment kill their dream. Today, we can decide to eradicate the stupid, senseless poverty that will see 3 million people die this year in Africa because of malaria - a totally treatable disease. As Nelson Mandela says: "Sometimes it falls on a generation to be great. You can be that great generation." Give up? We've only just begun.

j.hari@independent.co.uk