United States, China, Russia and Germany. This aforementioned nation's are seen as the most powerful nations of the world they all have a strong economy, strong economy is the beginning of a powerful nation.
Rich economy = better education = better overall living conditions = more budget for infrastructure = more budget for military power
Once a country gets their economy going then everything just follows suit. When they have the money they can pretty much do anything to be able to improve on their country whether that is to strengthen their defense or just make sure their citizens are being treated well.