Personal and ideological differences may underpin the divergent approaches to the Democratic Republic of Congo (DRC) by South Africa and Zimbabwe.
President Robert Mugabe of Zimbabwe favours military intervention to help the DRC's President Laurent Kabila contain a "foreign-backed" rebel offensive against his apparently floundering regime.
But President Nelson Mandela is firmly against military intervention, believing that, unlike diplomatic intercession, it will compound rather than solve the problem.
Their differences came to a head on Wednesday, when Mr Mandela publicly rejected Mr Mugabe's proposal for military intervention, made only hours earlier.
Differing tactical and strategic assessments may be important determinants in their opposing policies. But observers here believe that Mr Mugabe's pique at being overshadowed by Mr Mandela since his release from prison in 1990 is another - if less central - element in the political equation.
They point out that until Mr Mandela emerged from prison to reconfer international respectability on South Africa - a pariah state under apartheid - Mr Mugabe's Zimbabwe was the strongest black-ruled state in southern Africa.
Their perception is backed by reports in Zimbabwe's opposition press that Mr Mugabe was once miffed - to put it mildly - when an aircraft carrying Mr Mandela was given preference over a Zimbabwean plane in Lesotho at the coronation of King Letsie III.
Ideology and history still serve as actual or potential irritants in relations between Mr Mugabe and Mr Mandela, and the parties which they led.
The natural ally in South Africa of Mr Mugabe's Zimbabwe African National Union is the Pan Africanist Congress (PAC), not Mr Mandela's African National Congress.
Zanu and the PAC were backed by China, not the former Soviet Union, in the liberation wars against white governments in Southern Africa. The ANC's historical links in Zimbabwe are with Mr Joshua Nkomo's Zimbabwe African People's Union.
While past differences should not be overstressed - Mr Nkomo is now a vice-president in Mr Mugabe's government - they cannot be ignored either, particularly in times of disagreement between the neighbouring states.
Professor John Stremlau of the University of the Witwatersrand cites another factor which helps explain the divergence.
Mr Mugabe heads an "increasingly authoritarian government" and thus faces fewer constraints in deploying troops in a foreign country, the professor says. By contrast, Mr Mandela heads a democratic government and has to account for major decisions, including - and perhaps especially - the deployment of soldiers outside South Africa's borders.
Referring to his clash with Mr Mandela on Wednesday, Mr Mugabe is reported to have told reporters yesterday:
"No one is compelled within SADC (the Southern African Development Conference) to go into a campaign of assisting a country beset by conflict. Those who want to keep out . . . fine, let them keep out. But let them keep silence about those who want to help."
The sharpness of his tone indicates that the differences between the two leaders are far from resolved.