The United Nations announced a milestone this week. For the first time, a UN-led force helped beat back a rebel group to protect a civilian population. Instead of its usual defense-only peacekeeping, the UN engaged in aggressive peace enforcement – with helicopters, snipers, and artillery. The Security Council authorized the unprecedented offensive last March for Congo. But the UN’s Force Intervention Brigade only began fighting in August, alongside Congo’s military, against a rebel force known as M23.