Brian Steidle's only weapons against mass killing were his pen, paper, and camera. The former Marine captain catalogs what they caught in Darfur, Sudan, with quick-fire urgency: toddlers with their faces smashed in, men castrated and left to bleed to death, charred bodies of villagers locked in huts later burned down. Charged only with monitoring ceasefire violations in the war-wracked region, he soon grew weary of playing spectator to genocide. So after six months, the 28-year-old Mr. Steidle returned to the United States a month ago and launched his own offensive to stop the killing. In mid-March he criss-crossed Washington,...