WADAKONA, South Sudan — As the United States considers lifting sanctions on Sudan, one of the most sensitive issues is on display in these tense borderlands: weapons. South Sudan’s government accuses its neighbor of supplying arms to rebels fighting its bloody civil war. Opposition fighters, who recently defected to the South Sudan government side, described how weapons flow in from Sudan — and how rebels flee there to find safe harbor. Past documentation by arms experts that Sudan has supplied weapons to South Sudan’s rebels are a concern as the Trump administration considers permanently lifting sanctions on Sudan in October.