The point, in an event sure to generate huge domestic media interest, is to provide Mr Clinton with an unusual opportunity to instil in the American public the perception that he has scored a foreign policy success.
The President of the African National Congress and the President of white South Africa will benefit politically back home when their photographs appear on the front pages. Images of the two, smiling and triumphant next to the world's most powerful individual, will displease their main rivals, the conservative Zulus of Inkatha and the far-right Conservative Party, and help to fix the idea among their countrymen that the election on 27 April 1994 will be a two-horse race.
The Philadelphia three should all come away from today's ceremony amply satisfied. The American public would be misled, however, if it concluded that its government had played a decisive role in forging the agreement on Friday by South Africa's two great historical antagonists, the ANC and the National Party. The fact is, as diplomats in Pretoria acknowledge, that the world's influence on South African politics has diminished considerably since February 1990, when President de Klerk unbanned the ANC and released Mr Mandela from prison.
Pre-1990 foreign governments had what diplomats call a 'wish list', the key points of which - release Mandela, repeal apartheid laws, start constitutional talks - the government was pressed into fulfilling. Today Mr de Klerk and Mr Mandela, who have met privately on countless occasions and have already entered into a de facto joint presidency, are always available to each other on the telephone. As for their chief negotiators, cabinet minister Roelf Meyer and ANC secretary-general Cyril Ramaphosa, they have become such good friends - they go trout-fishing together - that the new joke at the multi-party forum where negotiations take place is to speak of them as one man with one name, Cyril Meyer.
Today's ceremony in Philadelphia dramatises this special relationship, miraculous at a time when much of the world is beset by seemingly intractable conflicts. George Soros, the billionaire financier, remarked on a visit that South Africa had a fighting chance to sort out its problems where Eastern Europe did not. The key, he said, was the political convergence taking place at the centre. It began after Mr Mandela's release when, in a remarkable act of generosity, he declared that the aim was to find a middle way between black aspirations and white fears.
The ANC's opening bargaining position, broadly speaking, was to demand simple majority rule within a political system highly centralised in the manner to which South Africa had become accustomed during four decades of apartheid. The National Party's position was, through federalism and entrenched minority vetoes, to seek a system of power-sharing whose main, if unstated, aim was to avoid the possibility of blacks doing to whites what whites had done to blacks.
With painstaking slowness, each side has watered down its demands. At times in the past three years, as right-wing elements in the security forces and Inkatha's Zulu warriors unleashed their small wars on the black townships, it seemed as if negotiations were condemned to failure. Today the government and the ANC share in all essentials a common vision of the constitutional future. The ANC has accepted the desirability of conceding significant powers to regional governments. The government has dropped its insistence on minority vetoes. The deal is that the first democratic government elected next year will rule as a coalition government of national unity for five years. After that, majority rule will prevail.
Which is not to say that the deal is watertight. Danger still lurks on the extremes, specifically from those who opposed Friday's election date decision. Twenty delegations voted in favour, six against. Those six, united by their fear of losing privileges acquired under the apartheid constitution, are led by Mangosuthu Buthelezi's Inkatha and the far-right Conservative Party within what they call the Concerned South Africans Grouping (Cosag).
What gives Cosag its clout is that, while together they would be pressed to collect more than 10 per cent of the national vote, they have disproportionate access to guns and people who know how to use them. Chief Buthelezi, aware of where his power lies, has made thinly veiled threats about the example of Angola, where the Unita leader Jonas Savimbi went to war last year after he lost the election.
Whether the 'Savimbi option', as it is commonly described in South Africa, is taken up depends in the end on the security forces. The critical question is whether Mr de Klerk and Mr Mandela can persuade the most powerful sectors in the police and army to back democracy.
Here they will require the world's help again; the US, the EC and the UN will re-emerge as players, not only providing observers to ensure that the elections are fair, but also sending a message that the world needs an example of peace and reconciliation.