The people. If the citizens all decide to come together and form a democracy then that is what will happen. On the flip side the people can be tricked into fear and end up in a dictatorship such as North Korea. Their ability to participate in economic trade/production also influences the type of government and this spawns from where they are located. An example would be a smaller country will have less land therefor less total resources (assuming density of resources is constant, which in reality it isnt but this simplifies it)