The Prolegomena opens with the remark that we lack a systematic work dealing with international law (ius gentium, § 1), although classical textes provide numerous examples of how ancient Romans and Greeks undestood the question of a justly-waged war (§2-27). By providing a solution to this question Grotius wishes to help his times (§ 28-32): some of his contemporaries, after all, disparaged all use of weapons.
After presenting the themes of each of the three books (§ 33-35) Grotius explained how he classified his sources according to their usefulness in drawing up a groundplan of the natural elements of international law (§ 36-55) and he characterised the pronciples of the work (§ 56-61).