It depends on the metric you use. In economic terms, the United States is the largest economy followed by China, Japan, and Germany. (Note: If you count the European Union as a single entity, it has the largest economy. If you count GDP at Purchasing Power Parity, China has the largest economy.) In military terms, the United States is the largest, by far, though the margin has narrowed somewhat in recent years. Typically, this is followed by Russia, China, France and Britain - though the disparity between these countries is quite wide. For example, France and the UK have - especially after the 2008 recession - become quite small in numbers with this offset by the high quality of their equipment and training and that their forces have an independent global reach. China has huge numbers, but is relatively weak in equipment, training and technological sophistication. (Even these rankings are disputable and depend on certain assumptions of the utility of military power. For example, North Korea's military is largely viewed as weak, but because it has nuclear weapons, the country is well nigh invulnerable to attack except in the most extreme circumstances. The advantages of attacking it would be outweighed by the cost of doing so.) Bottom line, comparisons of international power are notoriously difficult to make. They are partly based on quantifiable numbers, part on rates of growth, i.e. who is growing and who is declining, and part on subjective perceptions. Think of this: In the early 1900s through WWII, Japan was considered one of the great powers. It defeated China and Russia in two relatively short wars. It was on the winning side in World War I. On December 7th, 1941, Japan attacked the U.S. and threw us half way across the Pacific - and it took us 4 years to claw our way back. Yet Japan had an economy only 1/10th the size of the United States. So, as you can see, measurements of power are somewhat relative. This site may be of interest: http://www.globalfirepower.com/