Big trips generate big hopes. During Bill Clinton's African tour five years ago, the buzzword was "renaissance". A new era was dawning, the talk went, for the world's most benighted continent. But within months it was being quietly forgotten.
Mr Clinton had barely left when Africa spiralled into a fresh cycle of turmoil. Old wars intensified, new ones started. Dictators entrenched themselves. HIV/Aids ravaged millions.
Now the superpower is visiting the powerless again. The visit will be good for President George Bush, honing his image as a president who cares for the poor. It will also briefly focus world attention on Africa's often ignored plight.
But will it leave a sense of change or just more hollow rhetoric?
One country not on the Bush itinerary is the Democratic Republic of Congo. More than 3.3 million people have died since 1998, in the world's most devastating conflict since the Second World War. Yet until a French deployment to Ituri last month, the Congo conflict was largely forgotten by the West.
Zimbabwe was southern Africa's breadbasket in Mr Clinton's time; now it is the basket case. The twin calamities of Robert Mugabe's rule and HIV/Aids have broken it.
One day in the capital, Harare, I met Elizabeth Simango sprawled on a dirty mat in a forlorn township. Close to death, the idea of life-saving drugs was alien to her. A few months later, she died.
Despite large strides by campaigners, those drugs remain beyond the reach of most of Africa's 30 million Aids sufferers.
The US is generous to Africa in many ways. It gives about £600m worth of food to the UN World Food Programme every year, helping to combat the periodic famines, and Mr Bush's pledge of $15bn (£9bn) for HIV/Aids has surprised critics.
But aid comes with small print. Critics fear the US will use its contribution to wring intellectual property concessions out of poor countries, and a row has flared about its use of GM food aid.
There are other contradictions. Despite the image of famine, large swaths of Africa are covered in rich pastures tilled by farmers. Yet due to US and European trade barriers, it is practically impossible for them to sell their produce abroad.
Oil remains a quiet preoccupation for the US President. He wants to tap the great reserves of West Africa. If he really wants to make a difference, he could make sure the oil revenues are spent well this time. That would be more valuable than all the platitudes about democracy and corruption.