What happen when new civilizations can into contact with each other

It is only one out of the two things. Either they make peace with each other, or they go to war with each other. If they make peace with each other, then they would share each other's culture, products, and many other things. Trade could also happen between these new civilizations which could lead to development. If they go to war with each other, they will be trying to conquer each other's land. One civilization will rule on top of the other.

The spread of new ideas: languages, weapons, trading, etc.