The Ibn Ezra explains that during war there are a lot of objects being lost so the posuk puts it here. It is interesting to note that many topics having to do with war are followed by pisukim discussing the care we should take with another jew’s money. During wartime you would think a penny is of no consequence, yet the Torah comes to teach us that Klal Yisrael, the holiest among the nations, must even then be very careful.