Aircraft were increasingly engaged in ground attack operations as the war wore on, aimed at disrupting enemy forces at or near the front line and during offensives. While formal tactical bombing raids were planned and usually directed at specific targets, ground-attack was usually carried out by individual pilots or small flights against targets of opportunity. Although the fitted machine guns were the primary armament for ground attack, bomb racks holding 20 lb Cooper bombs were soon fitted to many single-seat aircraft. Ground attack sorties were carried out at very low altitude and were often highly effective, in spite of the primitive nature of the weaponry involved, compared with later conflicts. The moral effect on ground troops subjected to air attack could even be decisive. Such operations became increasingly hazardous for the attacking aircraft, as one hit from small arms fire could bring an aircraft down and troops learned deflection shooting to hit relatively slow moving enemy aeroplanes.