What was the defining election of the 20th Century?

[PresidentSamTilden]

I posted in the 2016 forum recently, asking if this year's presidential election will be the most significant in recent times. A couple replies in there got me thinking about perspective. What could we call the defining election of the previous century, and why?

As a rough definition of "defining", I'll start with: An election that clarified the US' position and identity, with large and long lasting effect, setting a course that could have been radically changed with the opposite result. This was off the top of my head and is open to adjustment.

For the poll, I included some commonly cited important or divisive elections. Feel free to select other and let me know why. Discussion > forum poll results.

[PresidentSamTilden]

My real answer would be 1789, as each election led to the next.

Well, if Adams somehow won that, I'm pretty sure there wouldn't be a USA, lol.