The journalist and black nationalist leader Marcus Garvey wrote a poem about it. The reggae great Bob Marley sang about it. And the Libyan strongman Moammar Gadhafi poured his oil wealth into it. But none lived to see a United States of Africa.
This history of disappointed hopes will provide the backdrop in early August when President Barack Obama hosts the inaugural U.S.-Africa summit in Washington. Only a few of Africa's 54 leaders—including Zimbabwe's President Robert Mugabe, who is still the target of U.S. sanctions—haven't been invited.
The U.S. wants to discuss continent-wide issues, such as security and terrorism, and to promote regional initiatives, such as shared electricity. To stress the breadth of the meeting's aims, Mr. Obama plans to meet with the African heads of state as a group, not individually—a move that has ruffled some diplomatic feathers.
The vision of an impoverished continent of countries coming together as one, flexing its muscle in geopolitics and the global economy, has long enticed activists, poets and politicians. But today's Africa remains divided, largely along hastily drawn colonial-era borders.
The question now is whether the still-remote idea of political unity can find new life in the more modest goal of an integrated economic community.
The obstacles are formidable. Congolese women who trade eggs can't cross borders without giving away part of their load to officials and facing threats of sexual assault, according to a 2012 World Bank report; South Africa has feuded with Nigeria and Kenya over visa rules for their citizens; and a territorial row between Malawi and Tanzania over a lake separating them has hampered oil exploration.
"They hold hands, kiss each other—sometimes even shed tears," says one senior bank official who has attended pan-Africa summits in which leaders rhapsodize about fused economic futures. "Monday morning, there's nothing. It's all forgotten."
Some Africa experts warn that the Obama administration's effort to deal with the continent as a whole may be counterproductive, both diplomatically and strategically. "It's uniquely American. It's different. It's also high-risk," says Stephen Hayes, president of the Corporate Council on Africa, a Washington, D.C., trade organization.
The U.S. plays down such concerns. Mr. Obama will set aside "lots of time for the leaders during the summit," Assistant Secretary of State for African Affairs Linda Thomas-Greenfield said earlier this month, adding that protocol and security would be handled in a way that shows "respect for African leaders."
The more substantive question is whether this is the best approach for promoting U.S. interests on the continent. Several U.S. competitors have focused more on the immediate needs of individual African states than on the long-term possibilities of a united continent.
China, which devotes half of its $14.41 billion aid budget to the continent, regularly hosts individual African heads of state. At an Africa summit hosted by Japan in Tokyo in June, Prime Minister Shinzo Abe held 15- to 20-minute meetings with each leader, according to two Japanese diplomats in South Africa.
Historically, a united Africa has been more of an imaginative leap than a realistic prospect. Africa is larger than the U.S., China, India, Japan and Europe combined. The 5,000 miles of territory stretching from Tangier in the north to Cape Town in the south are home to more than a billion people, speaking more than 2,000 languages.
Perhaps unsurprisingly, the leading advocates for a united Africa have been romantics, visionaries or both. In his 1924 poem, "Hail! United States of Africa," Marcus Garvey saw unity as a way to liberate Africans from foreign repression. He tapped himself as "Provisional President of Africa," according to Colin Grant, the author of a biography of Garvey.
That dream was still alive in 1979, when Bob Marley released "Africa Unite," singing, "How good and how pleasant it would be before God and man/ To see the unification of all Africans."
As colonialism came to an end after World War II, many of Africa's new leaders argued that the continent's economic and social development required political unity. At a 1963 summit of African leaders, Kwame Nkrumah, the first president of newly independent Ghana, warned that if they "let this grand and historic opportunity slip by, then we shall give way to greater dissension and division among us, for which the people of Africa will never forgive us."
Perhaps the most vocal proponent of African unity was Libya's Col. Gadhafi. About 10 months before he was overthrown and slain in 2011, the dictator told an audience in Senegal that Africa should have a single navy to combat piracy. He also argued for a single African currency and lobbied for a united government (to be based in Libya).
Gadhafi won endorsements for a U.S. of Africa by supporting cash-strapped African leaders. He also picked up 15% of the African Union's overall membership dues, according to the Center for Strategic and International Studies, a Washington think tank. (The AU didn't respond to questions about Libya's financial contributions.)
At a 1991 meeting in Nigeria, African leaders did agree to unite the continent's smaller regional trading blocs. The treaty, adopted a few years later, set out a road map for creating free-trade zones, a continental customs union and ultimately a single currency for one massive market by 2028.
But Gadhafi had even bolder ambitions and pressured a group of African leaders, meeting in Libya in 2005, to set a deadline of 2015 for creating a government for a U.S. of Africa. Ugandan President Yoweri Museveni, who headed this effort at political federation, later shifted his stance, backing economic integration instead, according to his spokesman, Ofwono Opondo.
Foreign manufacturers have scant incentive to invest in Africa's bite-sized economies when their suppliers can't easily move goods across borders. East Africa's biggest economy, Kenya, is smaller than that of Madison, Wis., according to the Brookings Institution, a Washington think tank.
And because most African economies sell a lot of raw commodities and manufacture very little, they don't buy much from each other either. Intra-Africa trade amounts to just 12% of the continent's total trade, the AU says. In Europe, it is 60%; in North America, it is 40%.
Still, signs of integration are slowly surfacing and may help spur further growth. In West Africa, eight countries maintain a common currency plus a central bank; in Central Africa, six more do the same. East Africans can now travel within the region using only their national identity cards. East and southern African leaders aim to establish a customs-free trade zone this year.
The U.S.-Africa summit is likely to reinforce what some experts say has become central to Africa's future: Unity is no longer about ideology but economics. "Establish the veins through which the economy's lifeblood will flow," says Jakkie Cilliers, executive director of the Institute for Security Studies, a Pretoria think tank.
"No one leader can push African integration. Those days are gone—and they should be gone."
By PETER WONACOTT