How did trade and warfare make this
civilization strong?

The trade and warfare make civilization stronger because they promote the economy of civilization.

Explanation:

Whenever there is civilization there happens the production to meet their daily survival needs. later there exist a surplus good which makes them to export and to promote the trade.When there is a trade-in civilization so there exists a new way of improvement in civilization which also indirectly promotes the economy of civilization.  To safeguard the wealth and prosperity, we have to defend the enemy to defend them warfare is required. Thus it also preserves our wealth and makes our civilization stronger.