I translated this sentence from an old textbook of mine:
Praestat, inquiunt, si iam principatum Galliae obtinere non possumus, Gallorum quam Romanorum imperia perferre; I checked the translations of McDevitt and Bohn and they use the subjunctive form of possumus which I think makes more sense. This book I use is aimed at second year high school Latin students and frequently switches the grammar around and uses the indicative rather than the subjunctive as used in the original texts. Maybe the authors thought the students would understand the stories presented in the book easier but I find it more confusing actually.