The Proliferation Security Initiative (PSI) is an international effort launched in 2003 to stop the trafficking of weapons of mass destruction (WMD) and their delivery systems. It promotes cooperation among nations to interdict shipments of WMD-related materials and technologies, enhancing global security. This initiative reflects a shift in foreign policy toward a more proactive stance against potential threats, connecting closely with concepts like unilateralism and missile defense strategies that were prominent before 9/11.