Two months ago Obama decided to send a handful of troops to South Sudan. He also has sold weapons to the new country. The South has been independent only since last July but there are continuing disputes about the border.
In the latest dispute Sudan sent warplanes over a South Sudanese provincial capital. South Sudan's Information Ministry also reports that the planes bombed an oilfield in the area.
As usual each side accuses the other of starting the conflict. Sudan claims that the South Sudanese military is being backed by Darfur rebels.
South Sudan gained independence after a civil war that had lasted decades. Tensions between the two countries threaten another war. The Obama administration's decision to help out the South militarily could draw the U.S. into yet another war. For more see this article.