Howard Zinn, the Boston University historian who was a vocal critic of the Vietnam War and whose books, especially A People’s History of the United States, challenged students to rethink their childhood American history lessons, died Wednesday. He was 87. Zinn was traveling in Santa Monica when he suffered a heart attack. "He's made an amazing contribution to American intellectual and moral culture," Noam Chomsky told the Boston Globe. "He's changed the conscience of America in a highly constructive way. I really can't think of anyone I can compare him to in this respect."