Some topic ideas for turning points in American history:
Revolution era: Boston Tea Party (1773); Battle of Saratoga (1777)
Westward expansion: Discovery of gold in California (1848), Homestead Act (1862)
Civil rights: The Civil War (obviously), but also 14th Amendment (1868), Civil Rights Act (1964)
Women's rights: 19th Amendment (1920), FDA approval of birth control pill (1960),
Political transformation: The New Deal (1933 and following)
Loss of confidence in government: Vietnam War, especially the Tet Offensive (1968), Watergate scandal and resignation of Nixon (1972-74)
Global conflict: Collapse of communism and USSR -- end of Cold War (1989-91). 9/11 attacks begin the "war on terror" (2001)
Those are just some ideas, there could be ample others.