What country was most powerful at the beginning and end of World War I?

Answer

Not perhaps as clear cut as it may first appear. Economically, I suspect that in 1914 Britain may still have just been the strongest though it had been losing ground for some time to the United States and to a lesser degree Germany. It was financing the war that would virtually bankrupt Britain and put it heavily in debt to America and its own empire.

In terms of military power its even harder to judge. One of the main causes of the war had been the relentless arms race between the great powers of Europe and in particular a naval power struggle between Britain and Germany. In fact the alliance system that would tip Europe headlong into war had been developed partly because the main powers were so evenly matched - allies were essential to tip the balance. This parity of power can be seen in the way that when war did break out it immediately became a stalemate that neither side could break for four years.

By the war's end Germany was in economic ruin, France industrially devastated and with Britain heavily in debt to just about everyone. America became the most powerful nation in terms of economic strength, industrial might, manpower, and military.

Answer

Britain was the most powerful country at the beginning of world war 1 in terms of military might, Imperial colonies and 3nd largest economy (behind US and Germany), at the end of the war Britain was deminished but still considered one of the most powerful countries with an even larger empire navy as one of the victors of WW1, but was financially wounded, at the end of the Second world war is officially the end of British superpower and the Rise of Russia and the US, the United States became the only remaining superpower in the early 1990s with the dismantling of the USSR.

So your answer is Britain/Germany/ before WW1 and US/Britian after WW1