Distinguish international from non-international armed conflict with the applicable law.

The IAC: the armed conflict between states (the GCs' common Article 2: the declared war-or-any other armed conflict between two-or-more High Contracting Parties — including the occupation without resistance). The applicable law (the full regime): the four GCs + AP I + the Hague law + the custom; the combatants' full privileges (the POW status, the combatant immunity — U3.2's face); the grave breaches' machinery (U5.1). The threshold: any resort to force between states (no intensity requirement — the low-intensity border clashes included).