The civilization really started to flourish in the 4th Millennia BC when it started to spread out to other settlements such as Amri. All these new settlements were an extension of indigenous developments. Mohenjo-Daro grew as the capital of this early state and Harappa and Kalibangan emerged as subsidiary centers.
This was the political background of the state, but the real question is how did the civilization flourish? The one thing that has been uncovered is that armies were not central to the civilization. Trade, agriculture and animal husbandry were central to the whole civilization. Traces of growth of cereals such as wheat and barley have been found all over the civilization. And the inhabitants were one of the first to breed livestock. Seals showing images of cows and other livestock have been found by archaeologists. Also, the people of this civilization were the first to domesticate the fowl. "As to the domestic fowl, all species in the world over have been traced to the Indian Jungle Fowl." (Vohra, p.11) There have been signs of trade with western and central Asia as well as with the coastal towns of Gujarat. Another proof for this claim lies in archaeology. {During all the excavations over the years, only poor quality weapons have been found}. This shows that the empire did not maintain any proper armies of its own. {The conclusion we derive from this is that the empire was a cultural empire}. Even, the governance of the empire wasn't a monarchy as no fortifications, halls or any prestigious monuments have been discovered. .
The question then is, if the Indus Valley Civilization didn't spread through conquest then how did it flow? The answer is through trade and the general flow of ideas. This claim is backed by the fact that the methodology in architecture of these cities is strikingly similar. The similarities start from small details such as bricks. They, at first had similar dimensions but as seen later in Mohenjo-Daro, Harappa and as far as Kalibangan, they had the same exact dimensions.