Answer:
development and more needs
Explanation: as civilizations get more populous and get more populous they need more things to stay afloat. In the beginning their were 7 major civilizations which mainly traded but as they developed they needed defence againt their neighbors and help in defaeting their neighbors and with development comes more needs the people have but if one nation cant provide another can. Also with development comes a higher standard of living which means more things need to be imported and bought if one nation cant get it themselves so they might create a trade alliance like BRICS or the EU