World War II brought about a new era in atrocity, with the Holocaust, the atomic bombings of Hiroshima & Nagasaki, as well as massive casualties on all fronts from dozens of countries.

Wikipedia entry detailing the breakdown of statistics.

With such a huge loss of life, the tradition of a tragedy moved from the personal (think Shakespeare, individual struggles such as Othello or King Lear) to something new, something brought about by man's new-found capacity to annihilate one another on a global scale. The simplest connection is to the advances in the gears of war, weapons, vehicles, and namely the atomic bomb. But on a more philosophical level, the inherent capacity of man to bring about such violence developed alongside his tools.