Nuclear weapons have been have been hugely influential in shaping international

Nuclear weapons have been have been hugely influential in shaping international relations since 1945 even though they have not been used since the bombings of Hiroshima and Nagasaki that ended Word War II.   Why does Bracken believe the place of nuclear weapons in international affairs is shifting in the what he calls "the second nuclear age" and what lessons can be drawn from "the first nuclear age" that remain relevant in this changing landscape?