The US backed the secession of South Sudan led by Salva Kiir in 2011. Now it is threatening the South Sudanese president with a UN arms embargo as only China has increased its influence and arms sales there ever since. As Washington has pledged the US to checking Chinese ambitions in Africa, this was a setback.
The idea back in 2011, the year of Obama's Pivot To Asia, was to forge an independent US client state to better control oil flows in Central Africa. South Sudan contains 75% of Sudan's oil. The US had hoped that by retaining influence over Juba it would retain control over an important source of oil against China.
Given that Kiir aligned towards Beijing, especially after Vice President Machar decided to vie for power against “President for Life” Kiir back in 2013, the West has started to swing behind Machar while officially still being for Kiir. China's decisions to back Kiir with arms deals ties has made him an unreliable client for the US.
It is thought the US is pressurising Kiir to concede ground to Machar after two years of civil war so as to prevent not just the collapse of South Sudan into chaos but to retain the ability to assert oil interests against Beijing. It is also a question of US credibility after yet another failing 'humanitarian' intervention.
2011 saw not only US withdrawal from Iraq and the collapse of Syria into a civil war that has the great global and regional powers contending over pipeline routes in the Greater Middle East. It saw renewed attempts to check Chinese influence in Libya and across Africa where China threatens to the dominant player.
Whereas China tended to use infrastructure projects and investments to advance strategic resource interests, the proliferation of Islamist insurgents fanning across Sub Saharan Africa has provided both a threat and opportunity to ratchet up the expansion of counter-terrorist military technology such as drones.
China basically outmanoeuvred the US in South Sudan; they have managed to put Washington in a position of having to provide 27% percent of the cost of United Nations peacekeeping force and in humanitarian programmes while China reaps all the rewards in oil concessions and trade.
Obama in July 2015 made his first visit to his father's homeland in order to big up the US-Kenya trade and business relationship as well as counter Nairobi's shift away from its traditional role as a Western client state and the rise of Ugandan influence over South Sudan after its military intervention to prop up Kiir
The threat of regional power alignments in south-east Africa that exclude US influenced clients is it could lead to oil being pumped from Uganda and South Sudan through to the Indian Ocean and on to China over which the US would lack strategic control should military ties with Kenya were diminished.
The loss of US arms markets in strategic regions of Africa could well explain why big US defence corporations such as Lockheed Martin and Boeing are willing financers of George Clooney's humanitarian advocacy group in South Sudan, aptly called Sentry which is part of The Enough Project.