1. VIRGINIA TECH KILLINGS: Seung-Hui Cho, 23, who had avoided court-ordered mental health treatment despite a history of psychiatric problems, killed two fellow students in a dormitory on April 16, detoured to mail a hate-filled video of himself to NBC News, then shot dead 30 students and professors in a classroom building before killing himself. It was the worst mass shooting in modern U.S. history.

2. MORTGAGE CRISIS: A record-setting wave of mortgage foreclosures, coupled with a steep slump in the housing market, buffeted financial markets, caused multibillion-dollar losses at major banks and investment firms, and became an issue in the presidential campaign.

3. IRAQ WAR: The "surge" that sent more U.S. troops to Iraq was credited with helping reduce the overall level of violence. But thousands of Iraqis and hundreds of U.S. personnel were killed nonetheless during the year, and Iraqi political leaders struggled to make meaningful progress toward national reconciliation.