Jack Mendelsohn, a former senior Foreign Service officer, a member of the SALT II and START I delegations and a former deputy director of the Arms Control Association, discussed the new START treaty with foreign-affairs columnist John C. Bersia.

WASHINGTON — As I made my way from one national-security discussion to another, the conversation never strayed far from an imposing topic: the 21st century’s emerging or renascent powers. Whether Americans like it or not, they must share influence with several countries that have risen in prominence, notably China, India and Russia.