"Relations between Museveni and Kiir stem from a time when both were rebel fighters," says Daniel Large, Professor at the Central European University in Budapest and director of the research project "Sudan Open Archive."
After Museveni seized power in Uganda in 1986, he provided considerable support for the SPLM in their fight against the government in Sudan.
Since South Sudan's independence, the military of both countries have worked together, for example in fighting the "Lord's Resistance Army" militia.
Museveni has drawn criticism for his military involvement in the present crisis. "While on the one hand international mediators are energetically trying to convince the conflict parties of the necessity of a negotiated solution, Museveni is encouraging Kiir in his military action," Casie Copeland of the International Crisis Group (ICG) told DW.
Non-interference in internal conflicts is one of the primary principles of Chinese foreign policy. However Beijing is showing an unusually strong interest in events in South Sudan, observes Daniel Large.
Chinese foreign minister Wang Yi, who is currently in Addis Ababa, has called on both sides to accept an immediate ceasefire, adding that he was ready to mediate between them.
China's great interest in South Sudan can only partly be explained by the country's oil, of which China is the largest purchaser.
According to Professor Large, the significance of South Sudanese oil for China has been diminishing. The country had proved to be an unreliable supplier and a risky location for China's state-owned oil giant CNPC.
The fate of Chinese oil workers, who had to be evacuated in December after the fighting began, was followed closely by their compatriots. This, says Large, is an important reason for the Chinese leadership's decision to get involved.
In the years preceding South Sudan's independence, the Chinese Communist Party established close relations with the SPLM.
Good personal contacts exist with, among others, Vice-President Machar, and ex-SPLM Secretary General Pagan Amum, who is also allied with the rebels and is currently in prison in Juba.