Great Britain (Revolutionary War)
Great Britain (War of 1812)
The next few depends on if you consider the Indian nations independent or not
Mexico American War 1846
The Civil War ( only if you consider the Confederacy a country ) ( We lost more Americans in this war than all of our other wars obviously )
Spanish American War
All of the answers can be disputed ( as most events in history can ) because the many wars we had with Indian Nations. Look up Manifest Destiny. The US believed Westward expansion and territory to be its destiny. Our ancestors believed all the territory from the East to West coast was meant to belong to our nation.