Answer:
Geneva convention/Laws of war
Step-by-step explanation:
WW1 Didn't really have any rules when it came to war thus there were cruel weapons and tactics used causing many deaths on both sides of the war but in modern times there are many things that change the way you have to fight, such as ROE(Rules of Engagement) and certain ammo types must be used because the collateral damage they have is lessened compared to other rounds. Also technology has changed a lot since then, body armor, armored vehicles, and advancement in weapon technology making engagement distances far greater.