i was wondering about that, i mean technically it could be considered the last war because it was before the modern era (1900) and yes,i know about world war and etc but what i mean is that the mexican-american war was the last one that didn’t have a lot of planning into it, similar to wars in the older days,like there was no system control back then or like radio so technically, while not true, could it be considered “the last war of humanity”?
I don’t have a complete answer, but the mechanization of warfare should have a lot to do with it. Things like the semi-automatic pistol – they would have called it an “automatic” pistol during that era – the modern hand grenade, the airplane for reconnaissance (although lighter-than-air balloons might have been used? IDK), and other weapons wouldn’t have really appeared for the Mexican-American War (1846-1848).
Or did you mean the Spanish-American War (1898)? I think the same logic still stands, especially since train networks would have been more fully developed by the turn of the century.