Zinn's diplomatic visit to Hanoi with Reverend Daniel Berrigan, during the Tet Offensive in January 1968, resulted in the return of three American airmen, the first American POWs released by the North Vietnamese since the U.S. bombing of that nation had begun. The event was widely reported in the news media and discussed in a variety of books including "Who Spoke Up? American Protest Against the War in Vietnam 1963–1975" by Nancy Zaroulis and Gerald Sullivan. Zinn and the Berrigan brothers, Dan and Philip, remained friends and allies over the years.