Late 1920s: Adolf Hitler sets his ethnic-cleansing campaign into motion. His anti-Semitic propaganda helps gain the support of many Germans for his Nazi Party.

Today: Following terrorist attacks on New York City and Washington, D.C., on September 11, 2001, the world experiences several other attempted terrorist attacks.

Late 1920s: The stock market crash of 1929 marks the onset of the Great Depression. Although the causes of the crash are heavily debated, at least some blame goes to the massive amount of speculative trading in the 1920s, which was supported by the optimistic and naïve view that the good economy would continue to improve.

Today: The crash of overinflated technology stocks, which many people had thought would continue to rise in value, helps initiate a recession in the United States. The economy gets worse as much evidence of rampant corporate corruption is brought to light...