Ugh ... I'm pretty sure it was on one of those grey side pannels. Searching for it now. While we wait, here is something from the old Traveller Library data: One prohibition is clear and firm: use or possession of nuclear weapons, if discovered, and regardless of size or type, will almost certainly trigger Imperial intervention. The Imperium alone retains the rights to such weapons, because of their extreme destructive powers and the relatively low tech level at which they can be manufactured. For similar reasons, certain other weapons (chemical and bacteriological agents, and meson accelerators, for example) are strictly controlled, although they are not subject to the sweeping restrictions placed on nuclear weapons P22 Central Supply Catalog Prohibited Items
Some items are subject to a blanket prohibition. Special permits are
required to possess such items and stiff penalties are in place for
those who break these laws. In the Imperium, even governments
are subject to these laws and must account for all prohibited items.
The Imperial Bureaucracy conducts random audits to ensure
compliance.
Examples:
Nuclear weapons.
Antimatter power systems.
Chemical and Biological weapons other than non-lethals
(tranquilisers and tear gas).
Starship-grade meson guns and particle accelerators.
Electromagnetic Pulse weaponry capable of large-scale effects EDIT: Here it is! P176 Main Rulebook: Imperial Rules of War The rules of war are an accumulation of unwritten concepts established on a case-by-case basis. They have not been officially codified to prevent formal precedent from
preventing Imperial intervention. The main aim of the rules is to maintain the economic and military well-being of the realm.
The Imperium tolerates the use of force as a necessary outlet for built-up political and social pressures beyond the opponents’ ability to mediate. A short war is deemed
preferable to continuing tension, sabotage, political agitation, etc. However, attempts by extra-planetary forces to seize control of a world’s affairs are beyond the scope of
the “safety valve” rationale. Recognizing that often some community of interest exists between a faction or state on a planet and some off-planet organization, the Imperium
tolerates ‘assistance’ as long as it is deemed appropriate to the level of legitimate interest in the affairs of the world held by the extraplanetary organization. When it has
appeared that the primary burden for the conduct of the war has been carried by an extra-planetary power, the Imperium has intervened.
One prohibition is clear and firm: use or possession of nuclear weapons, if discovered, and regardless of size or type, will almost certainly trigger Imperial intervention. The
Imperium alone retains the rights to such weapons, because of their extreme destructive powers and the relatively low tech level at which they can be manufactured.
For similar reasons, certain other weapons (chemical and bacteriological agents, and meson accelerators, for example) are strictly controlled, although they are not subject
to the sweeping restrictions placed on nuclear weapons.