I think that the trench warfare that was dominant during World War I did a lot to do away with any visions of war being romantic or grandeur. The fighting in the mud of the trenches and the diseases that went along with it shattered any of those visions.
In World War I, the technology of killing had advanced so much since the days of knights and swordsmen that there was very little chance of individual valor making a concrete difference in the war's outcome. The tactics of fighting had remained the same for centuries - charge the other line with all of your men and see who is left standing.
There's not much glory or glamour to be found living 24/7 in a muddy trench full of disease and trenchfot cases, waiting for the whistle that sends you charging into heavy artillery and machine gun fire. Very few men returned from that war with any illusion of its reality.
That is a good question. People have always glamorized war. Just look at any body of literature from Homer to Vergil to the Song of Roland. All of these people thought of war as an arena to earn fame and glory, and as a place to show prowess. However, this vision of greatness is easily shattered in view of the massive death. This fact alone shows the foolishness of glorying in war. Also there is another fact, which might be more important. There has to be a strong reason for fighting, that is, a clear idea of good vs. evil, which World War I lacked a bit. In other words, people needed to know that this war was worth dying for no matter what, even if it meant massive deaths. People certainly felt this in WWII and this is why they came home as heroes. This sociological and psychological perspective goes a long way in explaining why certain wars are popular and others unpopular.
The romantic vision of war said that war was a glorious thing. It envisioned brave men in beautiful uniforms going on cavalry charges. It envisioned men living or dying based on whether they were brave and skilled in battle. But this idea was totally wiped out by WWI.
In that war, living or dying was more or less random. If the artillery hit you, you died. It was just a matter of luck. Much of the fighting consisted of being in trenches in the mud and trying to kill people as they ran toward you. This was not very glamorous.
So this war with its machine guns and artillery and poison gas took all the glory out of war because it made the men into parts of a machine who lived or died based mostly on luck.
American's had not fought in a war in a longtime. The Cvil War had been the last real war. When soldiers were called to fight they ahd strong ideals that they would go off to war with weapons in hand and quickly kill the enemy. They would return home victorious and looking as good as they had when they left.
What the young men were not prepared to face was the barrage of weapons aimed at their destruction. Barbed wire made crossing pathways difficult and advanced technology in weapons made it all the more difficult. The machine guns tore men down by marking lines of bullets through their bodies. Poison gas caused lingering painful deaths, and the war dragged on. Neither side was able to make any significant changes for a two year period and the war seemed to linger. Visions of killing the enemy and returning home quickly died away along side the dead men resting in the mud ridden trenches.