Mongols dominated warfare under Khan.
According to votes US is the most powerful military. In 20th century? There is still was USSR. China grew powerfuful day by day. Britain was still in play. And how many soldiers were dead in Vietnam, Korea against primitive armies.Japan kept US quite busy most of WWII. All 20th century was a turmoil of war with such megapowers as Germany, Britain, USSR, China, Japan. Khan never met something like that opposition in life. If he could lived longer he would conquer Europe.