Sentence:  If the Hessian troops sent here willy-nilly by the Hessian Government to fight for England in the 1770 's were mercenaries , what shall we call the UN troops sent to the Congo willy-nilly by their governments to <WSD>fight</WSD> for the United Nations ? 
Meaning of the word marked with <WSD> tags:  be engaged in a fight; carry on a fight