There are two ways that a Great War can begin:

It's simply a system that makes big wars possible. It should also be noted that not every Great War is going to be a World War- you're not always going to have every Great Power jumping in when the system is activated, and these wars may end with nobody getting dismantled or end in stalemate just as often as before. You might get something similar to World War 1, but if so then it would be accidental. Such a simulation is next to impossible, due to all the pre-existing conditions that would be necessary for World War 1 to occur, none of which might exist after 60-70 years of game time with all the divergences from history that can happen in PDM. The Great Wars are not a way to simulate World War 1.

It also adds a system where a defeated power may be dismantled by the victors, transferring control of all its colonial holdings as well as mandating the release of civilized country cores within its borders. Any war between Great Powers will cause them to seek out new allies, and bring far more countries into the mix than is normally possible- exactly the same kind of conflagration of alliances which caused World War 1.

The Great Wars system is designed to raise the stakes in the later part of the game.