Countries used to think that invading others and getting more land and more people to rule over would make them more powerful. Some still think so.
But modern countries increase their influence through trade and their economy.

European countries used to control huge parts of Africa, and all it did was cost them money. Also, generally, people don't like being ruled by a foreign power. For a dictatorship that's OK. But for a democracy, having lots of people in your country that don't want to be governed by you is a big problem.