Throughout history, great and terrible events have often hinged on chance. Here, historian Andrew Roberts has assembled a team of his prominent colleagues, asking them to consider what might have happened if major world events had gone differently. Amanda Foreman, author of "Georgiana, Duchess of Devonshire, " ponders what might have happened if Lincoln's Northern States of America and Queen Victoria's Great Britain had gone to war, as they so nearly did in 1861. George W. Bush's former White House adviser, David Frum, considers a President Al Gore's response to 9/11, while Conrad Black wonders how the U.S. might have entered World War II if the Japanese had not bombed Pearl Harbor. Whether it's Stalin fleeing Moscow in 1941, as envisioned by Simon Sebag Montefiore, or Napoleon not being forced to retreat from it in 1812, as pictured by Adam Zamoyski, these essays posit a fascinating, sometimes horrifying parallel universe.