Some of the first European countries to acquire colonies in the "New World" (and elsewhere) were Spain and Portugal. They "discovered America" (as the euro-centric interpretation goes), and had several colonies all over the globe (Americas, getting to Japan before others, India, East Africa, etc.). It would seem obvious to conclude that in the 15th/16th centuries Spain and Portugal were some of the richest (and hence most powerful ountries in Europe, right?
However, by the start of the 20th century, UK, France, Germany, and others were now very powerful. The British Empire was massive, the French Empire included a lot of Africa, etc. So it would seem only obvious to conclude that they were now very rich and powerful countries.
What factors caused this to change? It seems obvious to think that once you're big, rich and powerful, that it's easy to stay big, rich and powerful. What factors prevented Spain & Portugal from remaining big & powerful? What prevented them from acquiring African colonies? What prevented them from becoming big players in WW1 (a war among powerful European states)?