Consider watershed:
an event or time when important changes happen in history or in your life
(Longman)
Example:
The 1932 election represented a watershedwatershed in American politics.
If a phrase is OK, critical juncture might work.
juncture: a point of time, esp. one made critical by a concurrence of circumstances
(http://www.thefreedictionary.com/juncture)
Example:
The 1930s was a crucial time of change in Latin America, a "critical juncture" in David Collier's and Ruth Berins Collier's terms.
(source on Google Books)