This chapter introduces the four major crises that defined the first two decades of the new millennium. The phrase “the long two decades” encapsulates a period marked by a succession of transformative events that profoundly reshaped American society and politics—beginning with the national unity in the immediate aftermath of the 9/119/11 attacks and culminating in the January 6th incursion into the U.S. Capitol, one of the most divisive moments in American history. The four crises of this period—the 9/11 attacks, the invasion of IraqInvasion of Iraq, the Great RecessionGreat Recession, and the COVID-19Coronavirus Disease 2019 (COVID-19) pandemicPandemic—draw striking parallels to four pivotal events of the twentieth century: Pearl Harbor, the Vietnam WarVietnam War, the Great DepressionGreat Depression, and the Spanish Flu. Each of these past crises similarly left an enduring imprint on the nation's political and social landscape. By examining the historical contexts, durations, and long-term consequences of these crises, this chapter explores both their shared patterns and unique distinctions, offering insight into how they have collectively shaped the trajectory of modern America.