The rule is simple: you ask what would happen if a historical battle (pre-1900, of course...) have different outcome than the OTL (losers become winners, and vice versa), then someone answered and mention other battle, and so on...
As an example:
Battle of Marathon, 490 BC, Greeks vs Persians.
Greece become Persian satrapy, Athens was destroyed, Western civilization ended.
So, my turn:
Battle of Hastings, 1066 AD, Normans vs English.