Under construction

History in some sense is defined by conflicts. They fall into a variety of categories:

  Political (Empire/Dynasty/Nation building)
  Social (Racial and Ethnic Constructions, Social and Economic Classes ) 
  Cultural (Religions – Belief Systems, Philosophies, and Ideologies
  Economic (Trade and Commerce – Labor Systems  – Capitalism and Socialism )
  Inequalities - Political, social, and economic
Paleolithic (2M - 12,000 years ago): all people had equal jobs because they were focused on survival, so the society was egalitarian pastoralists were nomads as well, so they were solely focused on survival.

Empires need to expand:
Territory was important for resources (food, water, ...)
To defend your territory you needed to expand your borders to natural boundaries.
  (Rivers, Mountain Ranges, Deserts.)
e.g. Roman Empire, Israel's annexation of the Gaza Strip, West Bank and Golan Heights. You need to protect your trade routes.
You need resources, gold, silver, slaves
You need to keep your army, built for defence, busy
Ego - Need to expand your power and influence

See Why do emperors expand their empires? - Quora

Mongolian Expansion 1206-1368 - Largest Empire in History. Genghis Kahn (1165-1227), Kublai Khan.
Some reasons:
Economic - Control of the Silk Trade route
Pride - They were good a fighting, incorporating superior technology and tactics.
Good Job - Soldiers were treated will and respected.  It was a good job.
They maintained their territory because  they were welcoming other cultures, religions and customs. 

See Why did the Mongols conquer so much land? - Quora

Source: Armed conflicts, 1946–2014 - There´se Pettersson & Peter Wallensteen -
Uppsala Conflict Data Program, Uppsala University

last updated 8 Oct 2017