Swiss banks, feminism, John Dean, more; Thinking About the Next War, by Thomas Powers. New York: Alfred A. Knopf. 158 pp. $10.95.

''Since 1945, the United States and the Soviet Union have been preparing to fight each other in a big war, and eventually they are going to do it. . . . We may want things to turn out differently, but wanting is not enough.''

Although Thomas Powers admits this is ''a bleak prospect,'' he does not shrink from thinking about it. In simple, moving prose he explores the likelihood and consequences of nuclear conflagration, speaking not only as a Pulitzer Prize-winning journalist but as the father of three young daughters.

These 18 quiet essays, published originally in Commonweal, combine careful reasoning, current events, history, and personal experiences. Although pessimistic, Powers is not defeatist. A move away from war ''must be personal before it can be political,'' he writes. ''It is worth taking for its own sake, whatever the results. We need no one's permission to begin.''