Weve been lucky that existing nukes have been under the control of relatively stable states that have agreed to and practiced cooperative restraint with them. What happens when more states, more unstable ones, acquire them for necessary defense? What happens when those regimes undergo violent revolutions/coups? When the leadership cant be trusted to be sane? How long will the world remain this lucky for? Nuclear proliferation will not lead to prolonged peace. It may seem that way for a time, up to the moment it leads to something worse than any previous world war.