This ties into the idea that a civilization could go to war over a variety of things; population expansion, resources, cultural differences, religious differences, ideological differences, and many more. For early human beings and even human ancestors, there has always existed this competition for resources such as food, water, etc. this is something that Charles Darwin noted in his conclusion of animals. This notion has existed in the form of an evolutionary hierarchy for animals and in the case of Social Darwinism, humans. This notion that there is a hierarchy system in the development of mankind is something that has regrettably plagued the Native Americans, Africans, Asians, and other ethnic groups aside from Caucasian.
Interestingly I am not landlocked, nor am a civilization that relies on warfare. Being landlocked is something that many countries fear, in that it limits their potential to expand. I am referring to an expansion in resources, borders, land, and other entities that could lead to the development of civilizations. Being landlocked implies that a nation is surrounded by other nations, and has no potential of evading that fate unless through diplomatic unification or warfare. Take for instance the nation of Switzerland or Luxembourg, both of them are minuscule nations when compared to Germany or France, and they are surrounded by other European neighbors.
As previously stated, warfare could be traced back to any number of events or circumstances and among them are politics and power. A civilization has the desire to expand its borders at some point in time, and typically it will achieve this goal in some manner of way. In the case of many monarchies in World History, they have achieved this through either political marriages, treaties, or simply purchasing land from the other. They have also sough out the usage of the sword or firearm to speed up this process of border/territory expansion.
In addition, power is something that is intertwined with the notion of warfare. many civilizations have utilized warfare to make their power credible to the world. An ancient example would be the early city-state of Rome challenging the massive naval power of Carthage, and winning the war against them. Although, a later example would be the tragic decision of the continued American involvement in French-Indochina (Vietnam), and the devastating defeat that the U.S. had been dealt. Some rulers believed that the only way to constantly grow ones nation was through the act of warfare, and it existed through Adolf Hitler, Benito Mussolini, and other world leaders.
It should be no surprise that Gajah has amassed such a massive army for his attempted conquest of my lands. This is a typical strategy for all warlike civilizations; mass produce your unique unit, throw in some archers and catapults and you are guaranteed a victory. Unless your facing off with the Shoshone, then that's a different story. In this case, while the Kris swordsman of Indonesia is a very powerful unit, it has a disadvantage in the Shoshone homeland. This is due to the defensive bonus that the Shoshone have.
Returning to the subject of the era that the war is taking place, it is important to note that it is in 210 B.C.. The reason that this is so important is because of the fact that neither he nor I have that much advanced technology to have a truly superior advantage. However, while military technology is equal in regards to research, Gajah has more units than I. This is due to his civilizations concept of warfare for advancing their people. Like other civilizations, the Indonesians' were conquerors and very powerful ones at that. Whereas the Shoshone prefer defense as opposed to offense. That is the way of war for my people, the best of offense is a fantastic defense.