In order to improve their fighting, observation, and reconnaissance capabilities, Bulgarian special forces and the Royal Marines are collaborating on combined training. 

Bulgarian special forces have been training with the Royal Marines in Scotland, marking Bulgaria’s largest UK deployment in a decade. The two units previously trained together on exercise in Bulgaria, but this visit aimed to strengthen the relationship by sharing knowledge and best practices. The Bulgarian troops, experts in irregular and clandestine warfare, are sharing their close-quarter combat expertise with the Royal Marines. In return, the Royal Marines are gaining insight into the Bulgarian soldiers’ tactics in built-up areas, while also sharing their own knowledge of surveillance and reconnaissance missions. The training is intended to enhance interoperability between NATO allies in the event of a crisis. 

Source: https://www.forces.net/services/royal-marines/bulgarian-special-forces-team-royal-marines-their-biggest-uk-deployment