To understand nations at war, you have to look at how their economies function. With World War II on the horizon, Europe and Asia dug themselves in for a fight - and a look at each other's resources told them what to expect. European economies were so closely connected that some people expected they have to avoid another world war or destroy their finances, but in fact World War I had taught them how to prepare for just such a scenario. Germany, France, and Great Britain all invested in their military before war broke out. When evaluating these economies to see how war would affect them, we look at four main factors: GDP, population, territorial extent, and per capita income. Broadly, this helps us determine how resilient, expansive, self-sufficient, and developed a nation is. All of those factors determine how a nation must conduct its war. For example, the vast territorial holdings of the British Empire meant that they had vast resources to draw upon but needed a long time to mobilize them, which helped Germany determine that they needed to strike fast and win big if they hoped to win the war before Britain's full resources came into play. Japan also estimated that they could win a war in the Pacific if they managed to win before the US had been involved for more than 6 months. These calculations drove the early strategies of the Axis powers, but the participation of the US would later prove to be a crucial factor.