There is actually a great, very readable.academic book on the topic called Supplying War by Martin van Creveld. Van Creveld wrote several books on similar issues. The simple answer is, armies weren't supplied, they followed supply. So, prior to modern transport, military engagements took place relatively close to rivers or other major fresh water sources, both to get the water and the food from farms near water sources. As you probably know, there were also campaign seasons when weather permitted more active militaries. Of course there were exceptions, but for the most part, military strategy had to take account of nature much more than it does now.