Sentence:  Had the situation been reversed , had , for instance , England been the enemy in 1898 because of issues of concern chiefly to New England , there is little doubt that large numbers of Southerners would have happily put on their old Confederate uniforms to <WSD>fight</WSD> as allies of Britain . 
Meaning of the word marked with <WSD> tags:  be engaged in a fight; carry on a fight