Since @bozz took my WW@ answer I'll go a different route
Waterloo
Think about what the world was like then and what would have happened if Napoleon had won at Waterloo.
Would Germany even exist? What would have happened after Napoleon died a few years later of stomach cancer and his 13 year old nephew Napoleon III would have assumed the throne.? Most likely a civil war would have ensued.
What would the world be like now with 200 years of chain reaction/alternate timeline from that single event and what followed?
Baguette and cheese anyone?
If Napoleon had triumphed at Waterloo, much of the world might be speaking French today
Ahh oui oui :)