Why is early childhood a crucial time for addressing physical aggression? when aggression is still high at the end of early childhood,

I believe the answer is:  it is a strong predictor of later aggression in adolescence and adulthood

Children who display constant physical aggression toward others generally experience some sort of problem in their home (they are either being ignored by their parents or they witness their parents do it and imitate them).
If this left unhandled, the aggression would become a habit as they grow up and make it much harder to be changed.

Ongoing civil wars western roman empire finally collapsed476 c.e. romulus, the last of theroman  emperors in  the west, was overthrown by the germanic leader odoacer, who became the first barbarian to rule in rome. the order that the  roman empire  had brought towestern  europe for 1000 years was no more.