This week in etymology I learned that the original Old English meaning of our word 'with' meant...against or opposed to! We still have this in words like withstand, withdraw, and withhold.
The word meaning together was 'mid' which we still see in words like midwife and middle.
The Viking invaders were the ones who changed that - their usage of 'with' suggested proximity (as in 'fighting with') and that eventually replaced the Old English 'mid.'