Explain with the Nuclear Tests cases.

The premier modern source: written agreements between states (or states and international organizations) governed by international law — codified by the Vienna Convention on the Law of Treaties (VCLT), 1969 (definition Art. 2(1)(a); pacta sunt servanda Art. 26; jus cogens override Arts. 53, 64). As source: particular (contract) treaties bind only their parties; law-making (normative) treaties declare or create general rules (the UN Charter's force-prohibition, the Geneva Conventions, UNCLOS) — the modern vehicle of codification and progressive development. Treaties are the most litigated "source" in Nepali foreign-relations law (the Article 279 ratification architecture — Unit 1; the law of treaties in full at Unit 9).