Related Rule
Practice Relating to Rule 4. Definition of Armed Forces
Section B. Incorporation of paramilitary or armed law enforcement agencies into armed forces
Germany’s Military Manual (1992) states:
Whenever a party to a conflict incorporates a paramilitary or armed law enforcement agency into its armed forces it shall notify the other parties to the conflict. In the Federal Republic of Germany the Federal Border Commands including their Border Guard formations and units as well as the Federal Border Guard School shall become part of the armed forces upon the outbreak of an armed conflict. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 307.
The Report on the Practice of Germany (1997) notes that from 1965 to 1994, German border guards were granted the status of combatants. In 1994, the German Parliament adopted a law that changed the status of the border guards. The reason for this change was that, as combatants, these guards could become legitimate enemy targets and they could involve local police forces as targets when operating in joint action. In addition, even civilian objects protected by the police might become targets. 
Report on the Practice of Germany, 1997, Chapter 1.1, referring to Federal Border Police Law, 1994, Article 4.