To attack or defeat an enemy; to destroy or avenge (archaic/literary usage common in historical contexts).
源氏の武将が敵軍を討つ場面は古典文学に多く描かれる。
Scenes of Genji warriors defeating enemy troops are often depicted in classical literature.
彼は国のために敵を討つ決意を固めた。
He resolved to strike down the enemy for his country.
復讐を誓い、主人公は仇を討つ旅に出た。
Swearing revenge, the protagonist set off on a journey to avenge the wrong.