He has developed several influential theories, such as the distinction between positive and negative peace, structural violence, theories on conflict and conflict resolution, the concept of peacebuilding, the structural theory of imperialism, and the theory of the United States as simultaneously a republic and an empire.
Ia mengembangkan beberapa teori penting seperti perbedaan antara perdamaian positif dan negatif, kekerasan struktural, teori konflik dan resolusi konflik, konsep pembangunan perdamaian, teori imperialisme struktural, dan teori Amerika Serikat sebagai republik sekaligus imperium.