If you have never read Kazuzo Kudo’s two books, JUDO IN ACTION, you should. His concept behind attack and defense is that defense should not be defensive in nature, but shaping the action for a counter-attack. In other words, your kaeshi-waza is actually an attack on your opponent’s attack.

Similarly, when you attack you are actually preventing your opponent from attacking you, so the attack also works as a defense. The goal is to shift your mindset from thinking defensively to thinking in terms of constantly attacking. By staying on the offensive you are keeping your opponent on the defense. This is also the principle behind good ippon judo.

From Mark Lonsdale’s PPT on Judo Tactics.