Id say the American civil war. It is the first modern war because it was, I believe, the first time the concept of total war was employed. The war was explicily waged against the enemy's civilians (not just to be brutal or to punish) but in explicit strategy to deprive the enemy of the the material needed to win.
Arguably the Napoleonic Wars. It was first instance in the history of warfare that you had battles fought by armies of 500,000 men and up. It was the first war to be fought on every single continent, expect Antarctica, and in all major oceans and it definitely affected all subsequent wars, not to mention global politics for a hundred years.
I would be inclined to say World War I, because you introduced motor vehicles, airplanes, flame throwers, chemical weapons, etc. Earlier wars relied on horse-drawn artillery.
Arguably the Napoleonic Wars. It was first instance in the history of warfare that you had battles fought by armies of 500,000 men and up. It was the first war to be fought on every single continent, expect Antarctica, and in all major oceans and it definitely affected all subsequent wars, not to mention global politics for a hundred years.
Umm.....I'm not so sure of that, TBH. For one, Australia(and even Oceania as a whole) hadn't even been settled all that much (by Europeans) yet. Secondly, does the War of 1812 even *remotely* count as an extension of that? As far as I can see, France had no real involvement in that conflict, and it was pretty much exclusively between us "Yanks" and the Brits. And then there's the matter of South America, most areas of which were still Spanish(or, in the case of Brazil, Portuguese) colonies. So I'm afraid that doesn't count, if we're using that particular standard.
(P.S.-Bolding is mine, btw)
And let's not forget Col. Emory Upton who, during Spotsylvania battle came with a smart idea of how to counteract the stopping action of trenches with the Schwerpunkt concept 50 years before the WWI western front.
I would be inclined to say World War I, because you introduced motor vehicles, airplanes, flame throwers, chemical weapons, etc. Earlier wars relied on horse-drawn artillery.
I's say the French Revolutionary Wars for the levee en masse and nationalism. And besides 1789 is a common year for the start of "modern history" classes.
In a stricter sense of the notion of "modern warfare" though, maybe the Russo-Japanese war would be a serious contender.