Sentence:  And if the foreigners <WSD>fighting</WSD> in the Katanga Army are mercenaries then Lafayette and von Steuben were mercenaries too , as were also the members of the Lafayette Escadrille in the early part of World War 1 , and of Chennault 's Flying Tigers in the early days of World War 2 , . 
Meaning of the word marked with <WSD> tags:  be engaged in a fight; carry on a fight