For me the most decisive, as in world changing, battles are Hastings, Saratoga and Teutoburgerwald - in no particular order...
Hastings
- Changed England's focus from Scanidanvia to Europe
- Introduced French into the Anglo-Saxon language
- Began the transformation that eventually changed the world - the British Empire and the latest lingua franca (yes, the irony is amazing).
Sarratoga
- Secured America's independence and formed what was to become the world's preemminent super power
- Forced England into focussing from America into Asia and, therefore, India was the replacement.
- Destroyed France's global empire ambitions, in effect.
Teutoburgerwald
- Defined Roman border in central Europe
- Created a schism that has remined until this very day - Latin/Germanic coutrnies
- WW1/WW2 fought along this schism
Yes, this is terribly eurocentric. Apologies as this further highlights my ignorance of the East...
Arguably, Tsushima is another as it was the first time a European power was fully defeated in a war, signalled Japans arrival as a super power and we all know where that led to...