Consider **watershed**:

> an event or time when important changes happen in history or in your
> life

([Longman][1])


Example:

> The 1932 election represented a **watershed** 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][2])


  [1]: http://www.ldoceonline.com/dictionary/watershed
  [2]: https://books.google.com/books?id=mBWiXGzBTN8C&pg=PA287&lpg=PA287&dq=%22The%201930s%20was%20a%20crucial%20time%20of%20change%20in%20Latin%20America,%20a%20critical%20juncture%22&source=bl&ots=ouskTFgjfB&sig=g_uWMs-2gZhU36oistJS3oyZd8Q&hl=en&sa=X&redir_esc=y#v=onepage&q&f=false