Rule 63. Use of the flags or military emblems, insignia or uniforms of neutral or other States not party to the conflict is prohibited.
Volume II, Chapter 18, Section G.
State practice establishes this rule as a norm of customary international law applicable in international armed conflicts and, arguably, also in non-international armed conflicts.
This rule is set forth in Additional Protocol I.
It is restated in other instruments, in particular the San Remo Manual on Naval Warfare.
The prohibition is contained in numerous military manuals.
Violation of this rule is an offence under the legislation of many States.
This includes the practice of States not party to Additional Protocol I.
No official contrary practice was found. No party has claimed the right to use the uniforms of neutral or other States not party to the conflict.
Military manuals which are applicable in or have been applied in non-international armed conflicts include this prohibition.
Violation of this rule in any armed conflict is an offence under the legislation of several States.
While no particular other practice was found with regard to non-international armed conflicts, no contrary practice was found either. No party to a non-international armed conflict was reported to have claimed the right to use the emblems or uniform of a neutral or other State not party to the conflict. It is very likely that the fact of implying involvement of a third State in a non-international armed conflict by wearing its uniform, for example, would be denounced by that State, as well as by the adverse party, as unlawful conduct. It can be argued therefore that there is a legitimate expectation that parties to a non-international armed conflict abide by this rule and that this rule is part of customary international law.