Agricultural techniques like irrigation could other civilization expand its borders because they could use irrigation to

Irrigation was one of the main reasons why civilizations were able to expand beyond their borders. The reason was that irrigation allowed them to not be dependant on rainfall for agriculture, which meant that they could farm more efficiently.

This new agricultural pattern meant that towns started having surplus food. And surplus food contributed to population growth, which allowed towns to turn into cities, and these cities to have a significant population that could become soldiers and wage wars of conquests.

Additionally, the growing population required more land to settle and farm, and this made them interested in conquering other territories.

interdependence is when all participating groups and individuals depend on each other. if you depend on one anothers goods, you end up trading in order to get the items you're looking for. (for ex: if i have food and you have water, we trade in order to depend on each other to live)