The Andean region that makes up the current country Peru is much larger than is readily apparent. Dwarfed by the Amazonian giant Brazil, it looks rather petite. But it is the combined size of California, Oregon, Washington, Nevada, and Montana.
The Wari Empire was on of a number of Empires that sprung up over time in Andean prehistory. The Wari are often cited as precursors to the Inca, and lasted much longer than they did (AD 600 to 1000). The Wari are credited with building the road network that the Incas later took as their own.
There is a fair amount of evidence that the Wari were a warlike people. At the very end of their history they were met by the giant fortress edifice of Kuelap; built by the Chachapoyan (Cloud People) as a citadel. This fortress contains 3 times more material than the larges of Egypt’s pyramids. Obviously the Wari were taken seriously.
As with many expansionist Empires their territory grew until they either ran into other large groups they were not able to defeat, or into barren territory that could not yield enough resources to make it pay. The Wari are a little different than some empires in that they appeared to bypass a considerable amount of material that they did not feel worth conquering. It is a little as if the Romans had marched through Germany (bypassing the Goths, et al) to conquer the Angles and Jutes in what is now Denmark.
At the end of the period the Wari ran into two groups, the Cajamarca and the Sican at the northern and southern edges of their territory. These groups appeared to be large enough or fierce enough that they halted their expansion.
An overlay of Wari Empire on the Map of Modern Peru
Kathatina J. Schreiber, from state to empire: the expansion of the Wari outside the Ayacucho Basin from The Origins and Development of the Andean State:
It is a curious aspect of most empires, at least those of the extensive type, that once they cease to expand they do not seem able to maintain themselves. This certainly seems true of the Wari. Furthermore, once this point had been reached, the economic and political organization of Wari had changed to such a degree (it was so geared to an expansionist economy, if you will) that not only did the empire collapse, but the Wari [homeland] state within Ayacucho Basin also collapsed, and Wari was abandoned. In addition those polities where tied to the Wari…also collapsed.