Guns.com: Today every squad of soldiers or marines has at least one fully automatic man-portable light machine gun issued to it. In 1918, this concept was foreign and a firearm that could fill this newly arrived at need was non-existent. Not to worry though, that most genius of American firearms engineers John Moses Browning, had something up his sleeve. The Army called it the M1918, but the troops just called it the BAR.
Prior to 1914, land combat was just a continuation of war as it had been in the time of Napoleon. When World War One erupted this illusion was rapidly shattered by quick firing mobile field artillery, long range bolt action rifles, belt fed machine gun and that grim reaper of a lost generation—mustard gas. These technologies broke the mobility of marching men and charging horses, exchanging open engagement for the soul depleting trench warfare that typified that conflict.
Source and read more: Guns.com