We start in 1789 with a "Tale of Two Nations": the newly born United States and France, a kingdom embarking on a radical transformation.

 

Here's another view of 1789, still in the center.  1789 launches France into a chaotic revolutionary period out which some equally chaotic republics (and other regimes) emerge. In the US, 1789 marks a bold, but peaceful transition to a new constitution which manages to still endure.

 

1789 is definitely f a starting place for the US.  Focusing on France, we can see it lurching through various constitutional frameworks while the US transferred power peacefully again and again. (The Civil War is viewed less as a "break" than as a failed rebellion.World War I (not represented) was a significant break in European History though not so much for France.  WWII was a break in both European History and for its colonies.

 

We don't know about formal democracies before Greece.  But it's development during the few generations before Pericles may suggest that it was a way of realizing some deep human needs that previous civilizations could only address in informal ways - if at all.

 

This is a deeper view of history and pre history.  Civilization as we know it is based on settled agriculture and the ability to generate and store surplus wealth.  No doubt there were informal ways of controlling "Big Men" including the constant threat of the  favored method of hunter/gatherers: murder

 

Complex human societies could not have developed until after the most recent glaciation. Before that there was little need for formal processes except those enshrined in custom and ritual transmitted mainly face to face and generation to generation.

 

Another look at the modern period.  Contrast the relatively bloodless "Glorious Revolution of 1688 with the tragic Paris Commune of 1871.  In the US power was transferred peacefully with only one major uprising in 1861 which was decidedly not intended to promote democracy. (The bloody US labor history can be referenced here.)

 

This  look at the twentieth century focuses on the US with its Progressive Era and then the New Deal.  In Western Europe more thoroughgoing Social Democratic reforms were instituted during approximately the same time-frame.