If there were two any armies some time in, say, 200 B.C.E. that wanted to go to war, how would they find each other? Was there just a huge array of scouts set up everywhere so that every possible point is within sight?
You can read the whole post here, where I've address it previously.
The long and the short of it is that armies do not exist to fight one another, they exist to secure objectives. Therefore one or more armies are on the move in pursuit of an objective and the other army attempts to block them. They are likely to run into one another based on inferences made from scouts and the only logical paths that armies can go.
The old adage is that battles take place along the lines of communication. This is as logical and simple a concept as you can imagine. It's easier to communicate with the people you need to communicate with, and there's a reason people use that path, well, for communication, so typically speaking you're going to see battles take place around areas that are easy and routinely traveled. Not necessarily on those sites, but around them. Armies can march through rough, rugged, shithole terrain (see Hannibal's march through the swamps to avoid engaging the Roman army, Alexander, B. (1993) How Great Generals Win p.41) but typically speaking this was dangerous, as seen when Hannibal lost a number of his men by risking those very same swamps. So typically, an army wants to use roads, rail, paths, trails, whatever guarantees their destination and ultimately makes life easier to keep a cohesive unit.
Is the first excerpt but I highly recommend reading the whole thing for a more in depth explanation of your answer.
Also to the others: I know, Trebia, it's just too late to edit that thread, lol.
hi! a few more related posts you might be interested in
How did large armies know where they would find one another in medieval times?
How do armies agree where and when to fight? Specially during the Crusades or Alexander's time?