I've heard (in an interview with German biologist Josef Reichholf) the argument that the Romans extended their empire as far north as they could grow their wine. A first glimpse at the map suggests that this could be true at least as a general rule. (The climate in Britain was warmer at the time.)
Would this then be a coincidence or could it be something that indeed had influence on the expansion strategies of the Roman empire? In particular, are there any relevant traces in contemporary (Roman) sources that point towards an answer?