Although this would never happen since we both share a native tongue and we have lots of treaties together this is all hypothetical speaking.


To determine this we will have to go over a few facts.


Well U.K. has roughly a population of 62,262,000.
US- 313,269,000

So theoretically if U.S. had to it could build a much larger military and in this day it all matters about numbers.

The U.S. military spends the most money of all countries in military expenditures much farther than any country in the world by almost 6 times. This is the list of top 5 military spenders in USDs so if you are from the U.K just convert it.

In Billions of Dollars:

Military of the U.S-$698
People's Liberation Army (China)-$119
British Armed Forces(U.K.)-$59.6
French Armed Forces(France)-$59.3
Armed Forces of the Russia Federation-(Russia)-$58.7


So that basically tells you that no matter what U.S will have the biggest and baddest army.



So in conclusion U.S would win against U.K.

Hypothetically speaking China and U.S the would be a more fair fight. China has a bigger population, but U.S spends more money on military funding.