9.6Challenges of the 21st CenturyAP United States HistorySeptember 11, 2001••••The War on TerrorThe Bush v. Gore election (2000)••••The Obama presidency•••••Political polarization•••••Ongoing debates•••••Key events summary••••••••••VocabularyUnit 9 topicsUnit 9 overview9.1Contextualizing Period 99.2Reagan and Conservatism9.3The End of the Cold War9.4A Changing Economy9.5Migration and Immigration in the 1990s and 2000s9.6Challenges of the 21st Century9.7Causation in Period 9← Previous9.5 Migration and Immigration in the 1990s and 2000sNext →9.7 Causation in Period 9Practice AP United States History questions →