Established terrorist groups, including the Irish Republican Army and the Red Brigades, have evolved into experts in urban terrorism, skilled individuals with military, political, and media understanding. Within the densely populated environments of today's cities, most terrorist organizations have come to rely on carefully placed time bombs as their primary modus operandi; this approach simplifies the terrorists' escape and maximizes public attention to the event. Bomb warnings are used by groups to summon emergency personnel to the scene, whereupon they detonate a second, more deadly device. The terrorist is skilled in the tactics of covert urban guerrilla warfare; the Third World often offers a socioeconomic and political climate conductive to urban terrorism. Public education and personal protection for persons at risk are two ways police try to defeat terrorists. Personal security is a matter of learning skills and attaining a certain attitude comprised of common sense and awareness of the methods used by terrorists.