Innovations in warfare that occurred during World War I:
Submarine warfare: The Germans introduced submarine warfare in 1915. Germany used submarines both to try to break Britain's blockade of Germany and to blockade Britain. Germany's submarines nearly won the war for her, but submarine attacks made the American people so mad that they supported President Wilson's desire to enter the war on Britain's side; the American navy became very decisive in the blockade of Germany. Without the ability to trade, provisions and raw materials for industry gave out, so that the German people became demoralized and the German army could not be adequately supplied. Previously, submarines had been very little used in warfare, because they had been too primitive and too few in number.
Gas attack was new: The German's introduced the practice of using artillery shells filled with poison gas to bombard an enemy position before the German infantry attacked that position. The British did not have so many gas shells as the Germans; one use the British made of their gas shells was to fire against German artillery, because this made the Germans put on their gas masks, and it was hard to work the guns with gas masks on.
By 1918, German infantrymen were trained to probe for and penetrate weak points in the enemy's lines and then to send reserves to exploit those points. Previously, infantrymen had been trained to try to crush the enemy's strong points, and reserves had been used to reinforce places on the line where they were being beaten back by the enemy. The new tactic was to follow the line of least resistance in order to penetrate through the enemy's line, to get behind the enemy, and to surround the enemy, and to do this rapidly. Before this time, the tactic had been to overrun the enemy from the front. (The Germans employed this new manuever more successfully in World War II than they did in World War I.)
On 15 September 1916, in the Battle of the Somme, the British put the first tanks into action. Two years later, they organized before Amiens, the first great armored breakthrough of modern warfare.
The use of the machine gun in World War I put into the hands of each small machine gun crew, the firepower of 40 riflemen. The machine gun had been used in previous wars, but it was too primitive and few in number to be of much significance in those wars. In World War I, the machine gun changed the nature of warfare, making infantry attacks across open ground very deadly and frequently unsuccessful. The infantry stayed in trenches most of the time for protection from the machine gun.
Indirect fire of artillery was of great significance in WW I. At the time of the American War Between The States, artillerymen had to see what they were shooting at. By the time of WW I, artillerymen could fire upon a target that was far out of their sight; they still required someone who could see the target, but he could now be far away from the guns, on a hill-top or near the front line, because he had a telephone and could tell the gunners how to adujst their sights so that they would hit the target.
The machine gun and the indirect fire of artillery made the battlefield such a deadly place for attacking troops, that trench warfare and stalemate (neither side making any gains) became the normal situation.
To try to overcome the machine gun and the indirect fire of artillery and the stalemate, both sides drafted men into their armies more than any combating nations had ever done before. Most of the able bodied, male popluation was drafted into the armies.
To supply these large armies, the whole industrial might of the nations at war was directed towards manufacturing, munitions, equipment, food, and other supplies.
Barbed wire was used to fortify lines against enemy infantry attack; I don't know if this was new in World War I, but it was extensive.
The use of fixed-wing aircraft as a weapon was probably new to World War I. The air plane was not prominent in WW I as it was in WW II, but it was used. In WW I, it was used first for reconnisance, then for attacking ground troops, then for bombing railroads and factories, but not in much numbers.
An article that describes a WW I battle, is Chapter 4 "The Somme, July 1st, 1916" in The Face of Battle by John Keegan, (New York: The Viking Press), 1976, pages 204-284. D25 K43.