Ukraine

Did killing Cesar at the end of Fallout New Vegas teach us nothing?

The war machine is too far along to be stopped easily.

Or if WW1 is your poison:

Initially, Wilhelm II wanted to halt the German mobilization, hoping for a peaceful resolution. However, Moltke insisted it was too late to stop the complex mobilization process once it had been set in motion. He argued that it was "like a wound-up clock" that couldn't simply be unwound. His words essentially conveyed that the mobilization machinery, once started, would continue inexorably towards war, effectively removing any possibility of reversal.