JUBA, Sudan — Only six years ago the Sudan People’s Liberation Army was a ragtag group of guerrilla fighters battling a bloody civil war with Sudan’s north. Next weekend, when the south breaks away and becomes the world’s newest country, the SPLA becomes a national army. The U.S. is investing tens of millions of dollars into this fledgling military, one that is massing troops on the internal north-south border as tensions — and violence — with the north rise.