What is the single military battle with the most far-reaching effects and consequences? What battle, if it had come out differently, would have had the most profound change on the march of history?
No brainer for me: Teutoburg Forest.
This ended Roman excursions into Germania. From then on, Europe was divided into to camps: Romantic and Germanic.
The ramifications of this were incomprehensible after the fall of the empire and rise of smaller, competing kingdoms and the ensuing conflict between the two cultures.