Before flare pistol was being invented, the United State have a hard time to see the enemy at night and it is difficult for them to attack their opponent without knowing landscape. So the flare gun was invented.
Flare pistol was invented by a U.S Navy named Edward Very, it was invented in the U.S.A during WW1, and to use the flare pistol you need to hold the gun and point it 90 to the sky then shoot it. different color of flare gun have different use, for example red and orange are used as emergency and white is to used it at night.
The flare pistol impact the war by using the white flare gun at night to illustrate the landscape and let the area bright for few moment of time so the sniper can attack any moving object. The good thing about flare pistol are it be a good tool if your ever get lost or stuck in a big area. But it is still a very danger weapon. Before it was being invented the army can't tell the attack and which is enemy and which is not, after it was being invented, many inventer get idea of flare pistol and invented flare thrower and some large area effect weapon and change the whole situation to careful at night to fearful at night.