So your question seems to be talking about the outbreak of a war, or within a war, why certain areas become the places where battles are fought. Some of the possible reasons why that occurs where it occurs:
Battle areas within a war:
... One side in a war sees a weak spot in the other side's defenses and chooses to attack at that perceived point of weakness.
... A particular area is an important, advantageous location to hold for military purposes, so a battle will be fought to gain control of that strategic location.
... Sometimes battles occur at certain locations because they are key population centers and the battle is intended to demoralize the opponent's civilian population. Sherman's March in Georgia (in US Civil War), the Germans bombardment of London in the Battle of Britain (World War II) or the atomic bombings of Hiroshima and Nagasaki by the US (World War II) would be examples of this.
Outbreak of a war:
... A territorial dispute over an area of land leads to fighting between the two nations or groups that seek to control that territory.
... A government is ruling in ways which cause strife and opposition among its people, and a rebellion or revolution breaks out against the government.
... Rivalry and egotism between leaders of different countries leads to one king or ruler leading his country to war against the other king or ruler. (Sometimes wars have happened for personal reasons by powerful men in charge of armies.)
