The correct answer is Civil War.
A Civil War best serve as a turning point between two historical periods.
A turning point is the precise, specific or significant moment when something starts to change.
For instance, any Civil War has represented a turning point in the history of a nation. The Civil War in the United States changed the society of America, ending slavery in the country. The Civil War in Mexico, called the Mexican Revolution, transformed the political in the country. The Civil War ended the 33 years of power od Dictator Porfirio Diaz. This represented the entrance of Mexico in a more modern period of social equity.