As a culture develops and grows, it's military must grow with it to protect it's people. In a tribal culture, there is usually to real organized form of military, though there are often warriors who are in charge of protecting people from marauding neighboring tribes. As tribes settled down and became agricultural societies there was more to protect. Not only did the increase of food allow there to be more people around, and therefore more people to protect, the food itself had to be protected. Tribes who had not perfected the agricultural side of things were often jealous of the surplus of food now enjoyed by others, and this food had to be protected. A more organized set of fighters had to be prepared, and a more advanced military came about. Along with this, tools and weapons could be crafted better and stronger for defense and attacks. Once a society developed into an Empire, this had to protect even more people, and even more resources. However, here things take on a more offensive flavor as the empire strives to expand itself. Military is trained to take over portions of land and societies of people to increase the power of an Empire.
Military involvement is Egypt was huge. The military expanded the borders of Egypt, taken in vast amounts of lands around it. They were innovative in the way they took lands, and dealt with other empires. Many of their tactics are still used today in military strategies, and many ancient militaries defined the borders of countries today.