KIEV, Ukraine, Jan. 16 - As protests here against a rigged presidential election overwhelmed the capital last fall, an alarm sounded at Interior Ministry bases outside the city. It was just after 10 p.m. on Nov. 28. More than 10,000 troops scrambled toward trucks. Most had helmets, shields and clubs. Three thousand carried guns. Many wore black masks. Within 45 minutes, according to their commander, Lt. Gen. Sergei Popkov, they had distributed ammunition and tear gas and were rushing out the gates. Kiev was tilting toward a terrible clash, a Soviet-style crackdown that could have brought civil war. And then,...