Hi I reckon this should be 'to draw first blood'. In a fight, it is to inflict a wound on your opponent first, before they can wound you.

Exactly what Neal said. Be the first to wound your opponent before they get you.

Rambo uses this phrase to defend his actions to his colonel. He is claiming self-defense, because the sheriff & deputies "drew first blood"---they started the battle by attacking Rambo first.

Hi Nominee, Neal is correct.This idiom is thought to have come from the world of boxing. The expression is also used metaphorically to mean, "to score a victory over someone" eg In the debate, it was the visitors who drew first blood. We were hoping to draw first blood. Unfortunately, we didn't. "Draw blood" has a third meaning as well. When you "draw blood" you succeed in hurting someone's feelings.eg The boy's scathing remarks clearly drew blood. eg Sam's remarks were intended to draw blood.