Rule 107. Spies
Rule 107. Combatants who are captured while engaged in espionage do not have the right to prisoner-of-war status. They may not be convicted or sentenced without previous trial.
Summary
State practice establishes this rule as a norm of customary international law applicable in international armed conflicts.
International armed conflicts
The rule that combatants engaged in espionage have no right to prisoner-of-war status and may be tried is a long-standing rule of customary international law already recognized in the Lieber Code, the Brussels Declaration and the Hague Regulations.[1]  It is also set forth in Additional Protocol I.[2] 
Numerous military manuals specify that combatants engaged in espionage have no right to prisoner-of-war status and that they may be regarded as spies.[3] 
No official contrary practice was found.
Definition of spies
It is also long-standing practice already recognized in the Lieber Code, the Brussels Declaration and the Hague Regulations that espionage is defined as gathering or attempting to gather information in territory controlled by an adverse party through an act undertaken on false pretences or deliberately in a clandestine manner.[4]  The definition includes combatants who wear civilian attire or who wear the uniform of the adversary but excludes combatants who are gathering information while wearing their own uniform. This definition is now codified in Additional Protocol I.[5]  It is set forth in numerous military manuals.[6] 
In addition, this rule applies only to a spy captured in the act whilst in enemy-controlled territory. The Brussels Declaration and the Hague Regulations recognize that a spy who rejoins his or her armed forces and who is subsequently captured must be treated as a prisoner of war and incurs no responsibility for previous acts of espionage.[7]  This rule is also set forth in Additional Protocol I.[8]  It is recognized in a number of military manuals.[9] 
Right to fair trial
A spy taken in the act may not be punished without previous trial. This requirement was already recognized in the Brussels Declaration and the Hague Regulations.[10]  It is also set forth in a number of military manuals.[11]  Captured spies are entitled to the fundamental guarantees set out in Chapter 32, including the right to a fair trial (see Rule 100). This is emphasized in the military manuals of Canada, Germany, New Zealand and Nigeria.[12]  It is also laid down in Additional Protocol I, which states that anyone who is not entitled to prisoner-of-war status, and does not benefit from more favourable treatment in accordance with the Fourth Geneva Convention, still enjoys the fundamental guarantees of Article 75 contained in Additional Protocol I.[13]  Consequently, the summary execution of spies is prohibited.

[1] Lieber Code, Article 88 (cited in Vol. II, Ch. 33, § 181); Brussels Declaration, Articles 20–21 (ibid., § 182); Hague Regulations, Articles 30–31 (ibid., § 178).
[2] Additional Protocol I, Article 46(1) (adopted by consensus) (ibid., § 179).
[3] See, e.g., the military manuals of Argentina (ibid., § 186), Australia (ibid., § 187), Belgium (ibid., § 188), Cameroon (ibid., §§ 189–190), Canada (ibid., § 191), Croatia (ibid., §§ 192–193), Ecuador (ibid., § 194), France (ibid., §§ 195–196), Germany (ibid., § 197), Hungary (ibid., § 198), Israel (ibid., § 199), Italy (ibid., § 200), Kenya (ibid., § 201), Madagascar (ibid., § 202), Netherlands (ibid., § 203), New Zealand (ibid., § 204), Nigeria (ibid., §§ 205–206), South Africa (ibid., § 207), Spain (ibid., § 208), Sweden (ibid., § 209), Switzerland (ibid., § 210), United Kingdom (ibid., §§ 211–212), United States (ibid., § 213) and Yugoslavia (ibid., § 214).
[4] Lieber Code, Article 88 (ibid., § 145); Brussels Declaration, Article 19 (ibid., § 146); Hague Regulations, Article 29 (ibid., § 143).
[5] Additional Protocol I, Article 46(2) (adopted by consensus) (ibid., § 144).
[6] See, e.g., the military manuals of Argentina (ibid., § 149), Australia (ibid., §§ 150–151), Belgium (ibid., § 152), Cameroon (ibid., § 153), Canada (ibid., § 154), Ecuador (ibid., § 155), France (ibid., § 156), Germany (ibid., § 157), Kenya (ibid., § 158), Netherlands (ibid., § 159), New Zealand (ibid., § 160), Nigeria (ibid., § 161), South Africa (ibid., § 162), Spain (ibid., § 163), Switzerland (ibid., § 164), United Kingdom (ibid., § 165), United States (ibid., § 166) and Yugoslavia (ibid., § 167).
[7] Brussels Declaration, Article 21 (ibid., § 182); Hague Regulations, Article 31 (ibid., § 178).
[8] Additional Protocol I, Article 46(4) (adopted by consensus).
[9] See, e.g., the military manuals of Argentina (cited in Vol. II, Ch. 33, § 186), Canada (ibid., § 191), Ecuador (ibid., § 194), Israel (ibid., § 199), Kenya (ibid., § 201), Netherlands (ibid., § 203), New Zealand (ibid., § 204), Nigeria (ibid., § 206), United Kingdom (ibid., § 212), United States (ibid., § 213) and Yugoslavia (ibid., § 214).
[10] Brussels Declaration, Article 20 (ibid., § 182); Hague Regulations, Article 30 (ibid., § 178).
[11] See, e.g., the military manuals of Argentina (ibid., § 186), Belgium (ibid., § 188), Canada (ibid., § 191), Germany (ibid., § 197), Kenya (ibid., § 201), Netherlands (ibid., § 203), New Zealand (ibid., § 204), Nigeria (ibid., § 206), Switzerland (ibid., § 210), United Kingdom (ibid., §§ 211–212), United States (ibid., § 213) and Yugoslavia (ibid., § 214).
[12] See Canada, LOAC Manual (ibid., § 191); Germany, Military Manual (ibid., § 197), New Zealand, Military Manual (ibid., § 204) and Nigeria, Manual on the Laws of War (ibid., § 206).
[13] Additional Protocol I, Article 45(3) (adopted by consensus) (ibid., § 180).