Once again, the world is faced with a mass-killing situation-U.S. leaders have called it "genocide." And again, like Rwanda in 1994 and so many other cases, the international response to Darfur's plight has been humanitarian assistance, condemnation, demands for the killing to stop, and a peacekeeping force with neither the means nor the mandate to defeat the killers. This response has neither convinced nor forced Janjaweed militiamen or their patrons in the Sudanese government to stop their assault on defenseless civilians. Even though the chance for action to prevent mass killing in Darfur has come and gone, we can learn from this tragedy what it will take to back up the pledge of "never again."