The major impact of technology on World War I (WWI) was that it made the war much more difficult for the infantry soldiers who did most of the fighting. The new technologies led to trench warfare and the lack of new tactics led to massive slaughter at the hands of the new technology.
New technology in the form of machine guns and rapid-fire artillery gave a huge advantage to any army fighting on the defensive in this war. When the war settled down into stalemate on the Western Front, each side had really good defensive positions with their machine guns and their artillery. Even so, both sides felt the need to launch offensives at times. This meant that they were going to hurl infantrymen at the entrenched enemy with their technological advantages. This led to massive slaughter.
With the war bogged down, another new technology made war even more miserable. This was poison gas. Both sides felt that the use of poison gas would allow them to break through the enemy defenses, but no such thing happened.
Technology also had other impacts. Technology in the form of tanks and airplanes helped to eventually end the conflict. However, the main impact of new technology was to make this a war that resulted in huge numbers of casualties due to the advantages enjoyed by defenders.
Technological advancements prior to the war set the stage for the tactical doctrine that would characterize the conflict. The new tactical needs of the War meant that the battlefields of the First World War served as a proving ground for a variety of weapons. The aftermath of the wholesale slaughter inflicted by these new technologies produced the Geneva Conventions.
The introduction of rifled small arms and artillery had altered the nature of warfare in the 50 years leading up to WWI. Prior to the widespread distribution of rifled weaponry, warfare relied more on maneuver warfare and the violence of action. Napoleon's success in the 1810s was due to his ability to out maneuver and manage his resources to overwhelm his enemy. In the 50 years after his campaigns, machining developed so that rifles could be produced in great numbers, and by the American Civil War in the early 1860s, rifled small arms and artillery forced an early shift away from Napoleonic doctrine toward a focus on fixed-positions. The accuracy of rifled guns compelled combatants to dig in, and early trench networks, like those outside of Petersburg, VA in the American Civil War, set the stage for the fixed-position doctrine that would be such an iconic feature of WWI. The American Civil War not only established the dominance of the the rifle, but technological developments made during the conflict brought the breechloading and repeating rifle to bear as weapons of warfare. The rapid-fire machine gun was also born out of this conflict, and though they were too late to heavily influence the outcome of the Civil War, the technology continued to develop and was a fixture on the fields of WWI from the earliest days of the German invasion.
The lessons learned on the bloody fields of Petersburg were late reaching the Continent, and it took time for WWI tacticians to realize the importance of the trench. In 1914, at the Battle of Mons, the British Expeditionary Force was able to engage the first wave of German attackers, as they advanced in a closed formation at ranges up to 1000 yards with repeating rifles and machine guns. German casualties were disproportionately heavy in the opening hours of the battle.
These early tactical blunders were remedied by a shift of doctrine into entrenched warfare. The German advance across Belgium and into France was stopped at the Battle of the Marne. Combined forces of British and French troops forced a halt to the German advance on the outskirts of Paris, and the Germans set about defending the territory gained in the previous months of campaign.
Thus began the protracted trench campaigns that characterize WWI. Both sides set about developing technologies to try to pry their enemy out of their positions and land a decisive blow. The tank, war zeppelin, combat aircraft (both bombers and fighters), flame thrower, and an array of lethal and non-lethal chemical weapons were all developed to accomplish this end. These tools of war, coupled with massed infantry offensives against these entrenchments, produced the massive casualties associated with the battlefields of the First World War. The five months of fighting at the Somme produced 1 million casualties on both sides. The 5 years of war produced an estimated 40 million military casualties, mostly due to the technological advancements produced on the fields of battle.
At the end of the conflict, the Geneva Conventions were adopted to set rules about the kinds of technologies that could be used in battle, in an attempt to minimize the slaughter in future conflicts.
It made war a lot more dangerous and new tactics formed