NAIROBI, Kenya, May 22 - The United Nations, burdened by its inability to stave off the mass killings in Rwanda in 1994 and by failed missions in Bosnia and Somalia, is allowing its peacekeepers to mount some of the most aggressive operations in its history. The change has been evolving over the last decade, as the Security Council has adopted the notion of "robust peacekeeping" and rejected the idea that the mere presence of blue-helmeted soldiers on the ground helps quell combat. It is most obvious in Congo, which commands by far the largest deployment of United Nations troops in...