From about 900 B.C. Assyrian kings sent out their armies to conquer new lands. Over 300 years Mesopotamia and lands further to the east and west became part of an Assyrian empire. Conquering foreign cities brought wealth.
Regions which had been conquered by the Assyrian army had to pay tribute to the king every year. If they rebelled against his rule, or refused to give tribute, the king would lead his army against them.
There were few armies who could defeat the Assyrians as they were well trained and they had good weapons.