 After 1970, some theorists claimed that nuclear proliferation made Clausewitzian concepts obsolete after the 20th-century period in which they dominated the world.John E. Sheppard, Jr., argues that by developing nuclear weapons, state-based conventional armies simultaneously both perfected their original purpose, to destroy a mirror image of themselves, and made themselves obsolete.No two powers have used nuclear weapons against each other, instead using conventional means or proxy wars to settle disputes.