Ammianus assessed Gratianus’ swift and successful war against Alamanni Lentienses as a remarkable and very useful achievement that came at the right time (Amm. 31. 10. 18). Some scholars believe, however, that it was both an unnecessary delay in helping the Eastern army to face Gothic hordes and a main factor to his uncle Valens’ defeat near Adrianople; such an assumption is still echoed by today’s scholarship. I will try to rebut this view and to defend Ammianus’ judgement in two ways. The former, by suggesting a ground-breaking interpretation of the Ammianean narrative (namely Amm. 31. 10. 1–17), which tells us about the lightning campaign of Gratianus much more than any other source; the latter, by reconstructing the actual crisis the Western Empire had to undergo in 378–379 AD. I also put forward a conjecture of my own on the brief lacuna of Amm. 31. 10. 4.