Complex sentence: Traditionally, before "fire-and-forget" ATGMs were used, the most effective countermeasure was to open fire at the location where the missile was fired from: either the operator would be forced to take cover or he would be killed.
Split into simpler sentences: Traditionally, before "fire-and-forget" ATGMs were used, the most effective countermeasure was to open fire at the location where the missile was fired from. Either the operator would be forced to take cover or he would be killed.