There tends to be very predictable places armies will go. Armies need good ground to cross, food and water to sustain it and ultimately end up at a place of stratetic importance. You get no where trying to haul elephants and carts of supplies up the side of a mountain. But there are known passages through the moutains (of which there are finite amounts) which would allow you to do so. Armies also have destinations. Hannibal was going to Rome. So there are only so many ways to move an army to Rome.

Once you've identified all the places on the map that an army could move to get to a place it wants to be. You send out scouts. Usually light riders. They go either range around the countryside looking for signs of the enemies passage (tracking) or approach (dust clouds for instance) or they find a spot with good sight lines and camp there until they spot something of note. Once they have something to report. They send word back to the commander.

He then looks at his map and given what has been reported, tries to work out where the enemy is going, by what route and how fast. Then determines where he should march his army to do something about it.

It wasn't all that uncommon for armies to accidentally miss each other there are plenty of times when the same army finds itself on two different parallel roads and completely missing the other half of itself. For example, during the Second English Civil War immediately before the King was beheaded and England declared a Republic an army of Scots and English Royalists where hit by a smaller force of Parliamentary troops, but the Royalists were spread out with the cavalry separate from the infantry. If could hook up then they'd be a bigger, stronger force than the not-quite-yet-Republicans arranged against them. Their infantry was beat and retreating southwards to link up with the cavalry and the cavalry was rushing north to pick up the infantry and smack the New Model Army. Problem is they were on parallel roads, so the infantry just kept running and the cavalry slammed into the New Model Army without support. Long story short that's it for the King and the War. And this happened in the 1640's.

It was super easy for armies to slip past each other in light woods or where rivers are involved. Just look at Nathaniel Greene's southern campaign in the American Revolution which basically took the form of being just barely too far away for the British to hit for years. The Mongols were also really good at not hitting armies except when they really want to.

The problem with all of this is two fold. First off, not all terrain can be traversed by a large number of men and wagons. For example, you can't haul wagons up mountains without serious problems that would pin you down and make you a sitting duck to hostile forces and you can't force hundreds of men to hike through a desert or swamp without lots of guys dying. Then there are the natural choke points, you know bridges and mountain passes where you kind of have to go. Then there's the other problem, which is what are you doing once you "get there". Usually the point of a war was the capture of land, which means they need to get a city or castle or something. Those things generally have walls that take a large amount of time to chew through. So, what happens when you are busy sieging a city or castle and that army equal to your army that is trying to stop you shows up? Well ask the Ottoman Turks in the Second Siege of Vienna when the Poles and Germans rolled up to bail the Austrian out (pro tip: You can't they're all dead). So, before you really start a siege that can take MONTHS which essentially guarantees that they will fight you when trapped between a wall and a pointy thing most army wanted to disrupt or destroy the other army.

Usually armies found each other by asking where the hell are they going/what are they doing? This is usually answered by war aims (going to the capital to kill the king) or by reports (they've just left heading north so they're going to York, I guess?). Once they have a general heading then they go that way asking locals and sending out scouts. It wasn't uncommon for armies to trip over one another on the road, but the most common one was to rush for the river crossing or mountain pass between the estimated location of the enemy and their destination and wait for them to arrive.

Remember you don't generally win wars by not fighting. Your whole point is to do something and that makes your moves predictable enough for them to send an army to stop you.

What podcasts have you been listening to?

You might be interested in this post: How did large armies know where they would find one another in medieval times?

(This information is from Mario Liverani's Book International Relations of the Ancient Near East) "War is seen as a legal procedure rather than competition over material advantage. The contenders must be on the same level, each having an equal chance of victory, victory should be due to merit (courage, valor) and not trickery. The battle must take place in a fixed time, “delimited space”, in an area known to both sides, open space suitable for army movement, with clear vision during the day. Battle takes place when both parties are ready, and when contact is made, best as frontal attack, the armies must fight." (109)

It's called a battle-field because often there were dedicated fields of battle that both sides knew. In the battle of Kadesh the Hittite King and the Egyptian King faced off after the Egyptian king travelled many weeks to reach the site of battle. During this travel the Egyptian army passed through a valley and they were not ambushed. This indicates that although the Egyptian pharaoh claimed that the Hittites did not follow the honorable conduct of warfare, they probably did. The site of the battle was next to a river with both armies having to cross to get to the site of battle, again indicating that some understanding about where the battle would take place existed.

The Egyptian pharaoh portrayed the Hittites as fighting unhonorably, and it is from these writings that we can deduce some of the official rules of war. Proper battle required a previous agreement upon the site of battle and that the two forces meet each other head to head. The timing of fighting was also supposed to be agreed upon and it was seen as dishonorable to use underhanded tactics. The final outcome of the battle should be decisive and would indicate the right side from the wrong side according to the judgment of the gods. This is what we know based on the Egyptian telling by inferring from their complaints against the Hittites.

So in short, often battle fields were agreed upon where armies would meet. This establishes a general area of battle around which the different parties can navigate to set up ambushes or change positioning. This only applies to battles between great kings and smaller bandits and groups used guerilla warfare rather than have big battles.

Follow up: How would an island nation (Britain) figure out where an attacker might land before modern technology?

May I ask what podcast you are listening to? It sounds interesting!

Could I trouble you for a link to the podcasts or a decent google search key?