What was the last major war, anywhere in the world, prior to the U.S. Civil War?
I believe the Napoleonic Wars were still winding down at the time of the US Civil War.
That is, of course, off the top of my head.
War of 1812? 
Last major war? I guess it depends on how major of a war you’re thinking.
I would say the Crimean War, fought from 1853-1856. There was also the (admittedly short) war of Italian Unification (April-July 1859). The last major war involving America would have been the Mexican-American war (1846-1848).
Here’s a good timeline: http://www.worldstatesmen.org/WARS.html
The Indian Mutiny or Sepoy Rebellion happen in 1857-58. It’s not on that page that Daver914 linked to for some reason. Not sure if you’d count it as a major war, but the same is true for the Italian Unification.
The Taiping Rebellion in China started in 1850 and ran through 1864. It was definitely a major war with approximatley 25,000,000 killed.
The Indian Mutiny is certainly a major conflict when you consider that more Victoria Crosses (Britains highest award for bravery in combat) were awarded during it, than during the Second World War. A fact that always amazed me anyway.
Of course, the standards for recieving the award might have changed in the interveaning years. I know quite a few Medals of Honor were awarded in the US Civil War for acts that wouldn’t be get a medal today.
That is correct. I don’t have a cite handy, but an entire regiment during the Civil War was offered the Medal of Honor as incentive to re-enlist. Congress recently rescinded a large portion of these rather dubiously-awarded Medals from the Civil War era.
The purge of “dubious” Medals of Honor was in 1917. The regiment you are thinking of is likely the 27th Maine. The Home of Heroes web site has a page on the “Great Purge”.