Why do great countries have great armies?

Great, or powerful, countries have great, or powerful, armies in order to keep their power. Military power is usually used to exercise one's power in either defensive and/or offensive terms. However, economic power also ties into being a powerful country. For example, the US has the world's most powerful army and the world's largest economy, and is the reason why the US is the world's most powerful country as a whole. However, although North Korea has the world's largest army, its economy is terrible, and this is one reason why North Korea is not such a powerful or influential country in the world.