This is a really interesting question, because as far as the US goes, this was a major part of their history. Between 1900-1945, there is a transition, between the British Empire losing its superpower status to the US. This is fueled by the recovery from the great depression, and the profits from the two world wars. Here is a period where we see a steady growth in the economic and political power of the US. At the turn of the century the army was small, inexperienced in modern warfare and weak. By 1917/18 it was supplying millions of troops to fight in the great war. Over these 45 years, the dollar started to become more dominant than the pound stirling. The US started to lead global decision making. This is the period of change, marked by the Great War (1914-1918) the Depression of '29-'33, and World War Two. Don't forget the physics revolution aswell, a key factor that would affect it's position in todays world. The leaders of space, nuclear and power.