Last September, a Ukrainian ship carrying a cargo of over 2,000 tons of weapons (including 33 T-72 tanks) to Kenya, was seized by Somali pirates, who promptly demanded $35 million for the ship. This was negotiated down to $20 million them finally, $3.2 million in ransom.
But while the negotiations, which took three months, were underway, questions were raised about where those tanks were headed. The paperwork said Kenya, and the Kenyans said the tanks were for their army. But the Kenyans use American and British armored vehicles. When the tanks finally arrived in Kenya, they were taken to an army base. But satellite photos showed that those tanks were moved several times, until they crossed the border into South Sudan. Two other ships carried more tanks to Kenya, and eventually about a hundred T-72s ended up in South Sudan.
What happened here is not unusual. Bribes were paid to get the tanks (and dozens of artillery pieces, plus thousands of tons of ammo, tools and spare parts) into Kenya and thence into South Sudan. All this was strictly illegal because of a UN arms embargo on Sudan. But the (largely Christian and black) people of southern Sudan have been fighting the (largely Moslem and Arab) people of northern Sudan for decades. There is a peace deal in effect between the Sudan government, and the southern rebels. This deal calls for a vote in the south, to decide if the people there want to remain part of Sudan, or split off and form a new government.
The big issue here is oil, which has been discovered near the northern border of "South Sudan." The Sudanese government is not trusted by the southerners, who expect an invasion if the vote calls for independence. The 2005 peace deal stipulated that the vote would have taken place by now, but has been delayed several times by the Sudanese government, and is now scheduled for April of next year. The South Sudanese are arming to maintain their official independence, in the face of a possible Sudanese invasion.
The Sudanese have been buying Chinese tanks, and have several hundred of them. Most are older models, about equal to the T-72s the southerners have. The Sudanese army has a bad track record when it comes to maintaining armored vehicles. So it is not expected to be capable of moving, and using, an overwhelming number of them in the south. Thus the new South Sudan tank force, if used by well trained crews, could prevent the Sudanese army from just blitzing their way through the south. The South Sudanese are getting covert help from Israel, and overt help from other Western nations. Sudan is an ally of Iran, and thus not very popular anywhere. That makes it easier for the South Sudanese to get along with a little help from their many new friends.