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, to either force the operator to take cover or kill him.
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. To either force the operator to take cover or kill him.