What was the most dominant military force in history?

Answer 1

For centuries, France and the UK were the greatest military powers in the world, and always fighting against each other.

Allied, the British and the French took Jerusalem in the Crusades, defeated Russia in the Crimean war, China in the Opium wars and Germany in WW1.

They also took control of big parts of the world with their colonial empires.

UK and France are the countries which had the greatest military power for the longest time.

Generally speaking, the Roman Legions probably hold the record for being dominant the longest over the largest amount of territory - from around 500 BCE to around 450 AD, covering practically all of Europe and North Africa.

In terms of being the most dominant force against contemporary opponents, that distinction goes most likely to the Mongol hordes of the 1200s, with a strong second going to the current-day U.S. armed forces. The Mongols were literally unstoppable for the duration of their conquests, easily defeating all Western and Mid-Eastern armies put in their way, and only encountering modest difficulty in conquering the majority of China. While modern U.S. forces can defeat any conventional opponent in today's world (and, more specifically, the US Navy dominates the world's oceans unlike any previous navy), they have significant difficulty with unconventional opponents, and thus are not quite as impressive as the Mongols. However, neither the Mongols nor the US have dominated for a very long time (~100 years for the Mongols, less than 30 for the US), so history will have to judge them on that, too.