It is too difficult to assess force, and the willingness to justify it comes in too many forms to treat it as a reliable guide to the international order. Balance works best if it is supported by agreement on generally accepted values. The balance of power prevents the emergence of opportunities to destroy the international order; agreement on generally accepted values prevents the emergence of a desire to destroy it. Power devoid of legitimacy provokes tests of strength; legitimacy devoid of power provokes empty posturing.