Machine Guns – Decimated attacking forces; made trench warfare deadly.
Tanks – First used to cross trenches and break stalemates.
Poison Gas – Chlorine, phosgene, and mustard gas introduced chemical warfare.
https://www.theworldwar.org/learn/about-wwi/spotlight-first-usage-poison-gas
https://cen.acs.org/articles/93/i8/100-years-chemical-weapons.html
https://www.warmuseum.ca/firstworldwar/history/battles-and-fighting/weapons-on-land/poison-gas/
https://www.poetryfoundation.org/poems/46560/dulce-et-decorum-est (Poem)
Airplanes – Used for reconnaissance, dogfighting, and limited bombing.
https://www.history.com/articles/world-war-i-aviation-airplanes
https://www.britannica.com/technology/military-aircraft/World-War-I
https://www.warmuseum.ca/firstworldwar/history/battles-and-fighting/air-war/command-of-the-air/
https://www.iwm.org.uk/history/voices-of-the-first-world-war-war-in-the-air
U-Boats (Submarines) – German submarines revolutionized naval warfare.
Artillery Advances – Heavy, long-range guns dominated battlefields.
Wireless Radio – Improved battlefield communication.