I think a good candidate would be the battle of the milvian bridge, involving constantine and maxentius. I'm sure peopel recognise this as an important battle, but i dont think that people give it as much attention as it deserves. This battle practically paves the way for for constantine to become emperor, but what occurs during the fighting is very important. he sees visions that tell him to conquer in the name of the christian lord. without this battle and constantine's rise to emperor, what would become of christianity? Its very possible that christianity would have stayed a minor religion, completly changing the history of the entire world, not just one region. you couldn't even imagine what the world would be without the influence of christianity, good or bad.