The question in the picture is actually very simple to answer because most of human history has been characterized by the fight of different groups of people over a territory. Land was until very recently, the most important economic resource because the economy of the world was based on agriculture, and this is why different groups of people fought so fiercely throughout history over it.
One example where the invasions of Germanic Peoples all over the Western Roman Empire during the Late Antiquity. These invasions resulted from the conflict between two different groups of people: the Germanic Peoples and the Romans, over a common territory: the lands of Western Europe.
The Germanic Peoples were victorious because they caused the fall of the Western Roman Empire, and replaced the Roman political order and rule with their own.
Many Japanese chose to serve the U.S. in World War II because they felt it was a way to repay them. Japanese Americans were at one time immigrants coming into the United States with little supplies and a dream or two. They found freedom in America, and because they were so thankful to find a place truly free, they fought for the U.S.
