War has always been hell. The "Honor and Glory" angle was perpetuated by the people who needed soldiers. They would lure in peasantry with those sorts of promises and, inevitably, most would die horrible deaths far from home or be unable to adapt back to civilian life. If you were on the battlefield, you likely lost most of your friends in combat. You were always hungry, often cold, and unbearably dirty with mud, blood, and insects. You were sick, in pain, and tired. Your clothes were always torn in one place or another and every day could be your last. Hell, more soldiers died in war from illness, the elements, hunger, or exhaustion than from an enemy blade.
If you were "lucky" enough to see battle, chances are you and your untrained comrades would be fodder for the enemy. Your job would be to serve as a distraction, break their front line, or absorb the brunt of their first charge/volley. Keep your formation, hold your ground, and you'll die. Break and run, and you'll die. Try to charge your enemy, be a hero, and you die. Even if, by the grace of whatever higher power you prayed to, you survived, you got to loot the dead and begin marching again for the next battle. If, by a mixture of luck and resilience you survived the war, you would carry physical and mental scars with you for the rest of your life that could kill you just as easily as a battle could.
And that doesn't even begin to cover what happened to civilians/noncombatants.
The reason why "War is Hell" is more prevalent nowadays is because of three reasons.
1. A cultural backlash against the glorification of war. Because "Honor and Glory" was the prevailing mood for the longest time, its only natural that we would begin to explore its opposite, to subvert the expected tropes and cliches.
2. We don't fight for beliefs as much anymore. You know who peddles the "Honor and Glory" myth nowadays? ISIS. When we go to war now, it is seldom about our religion or the fate of our nation. We fight for politics, or against nebulous foes. It is hard to see the honor and value in that sort of conflict.
3. Media. This was a major cause of change, actually. As photo journalism became viable, the images of War could be carried back to the common people. In the US, this happened around the Civil War, and the cultural baggage from that change has never truly left us. Without media as it is now, it would be harder to dispel the "Honor and Glory" myth. See Vietnam for just how powerful those images can be.