World War I (WW1)
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For starters, the loss of life was far greater than any war before hand known in history due to the military's use of new technologies, such as:
- War Submarines - used for destroying neutral ships delivering supply to their Allies.
- Machine Guns - capable of bringing down row after row of soldiers from a distance on the battlefield. This weapon, along with barbed wire and mines, made movement across open land both difficult and dangerous.
- War Tanks - The original tank, the Mark I was a heavy vehicle designed to flatten enemy fortifications. It was developed to be able to cross trenches, resist small-arms fire, travel over difficult terrain, carry supplies, and to capture fortified enemy positions.
- Airplanes - The main military role of aircraft in World War I was reconnaissance, says Jon Guttman, an historian of military aviation who's authored more than a dozen books about World War I aircraft and fighter pilots.
- Flamethrowers - an effective tool against fortifications and trenches, showering the enemy with burning liquid and flushing out troops who would be otherwise unassailable.
- Toxic Gases - The modern use of chemical weapons began with World War I, when both sides to the conflict used poisonous gas to inflict agonizing suffering and to cause significant battlefield casualties.
In summary to this, many other previous battles/wars in history lacked the availability of new technology in which was provided during World War I, which made matters worse for both sides during this war in the massive numbers of fallen souls on the battle fields. Hence, this was probably one of the greatest wars to have ever taken place in history, not including the aftermath of the following war, World War II, in which in the end placed 50 million more graves then the first World War did (which was 20 million dead).
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Hope this helps!