How do armies agree where and when to fight? Specially during the Crusades or Alexander's time?

You can read the whole post here where I addressed this before. The TL;DR of it is that armies fought out of necessity along lines of communication relative to their mutual objectives.

Let's look at this simply:

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.

So you'd have two armies doing their best to try to navigate terrain where they aren't terribly hard to see, and truth be told, armies leave a lot of mess behind, so properly trained scouts have a decent fix on where the enemy is roughly. Be this by spying an enemy marching down the road with your own eyes, or conferring logical information (a burned up, abandoned camp, discarded shells, tracks from the army, locals giving you information, horse shit, there are a thousand ways) scouts usually had a decent idea of where abouts an army was, if not necessarily where it was going.

So now we have logically dictated normal paths of armies, so how does this translate into two armies meeting and fighting one another? Typically speaking, a battle itself is not the ultimate goal of either commander, and can often times translate into one of the two commander's failures (not always, but sometimes). When a general, king, commander, what have you, is marching his army, it's to fulfill a set purpose.

So we'll use a simple, theoretically example plucked from my ass.

You have Gaius here, a legate fighting in Syria. His goal is not to go pick fights with local Syrians and then march around proudly because he killed 10,000 Syrians (he'd brag about that, but that wasn't his goal), the legate is trying to reach a strategic objective. In this theoretical situation we'll say he's trying to take the Seleucid city of Totallynotmadeupia. Why does he want Totallynotmadeupia? Because it is the political center of the current resistance, it has enough plunder to gratify his troops if it comes down to a brutal assault and that always looks good for Rome, but most important it's the central hub of where the Seleucid commanders are organizing their efforts. Grain, money, raw materials, weapons, and troop junctions all meet here in Totallynotmadeupia, so he has to go and take this city from the enemy, depriving them of all of the above.

Therefore, he will march towards it. Now given everything discussed, the Seleucids obviously don't want to lose Totallynotmadeupia, so they send out one of their armies which they are confident can defeat Gaius in battle. The goal here is that Gaius does not reach Totallynotmadeupia (heretofore referred to as Upia) with a sufficient force that he can take the city.

So now the Seleucid commander, we'll name him Nektos for fun, marches out with his army. His scouts tell him that Gaius has 10,000 legionaries, 2,000 auxiliaries (mostly foreign cavalry), and is roughly 8 days from Upia at his current pace. Scouts also report that Gaius is relying heavily on foraging because he's wandered away from an important supply train, hoping to catch the Seleucids unawares.

So Nektos now knows roughly where Gaius is, and Gaius gets a decent idea of where Nektos is, knowing that Nektos is coming out with a mixed force of mercenaries and hoplites that number about 15,000 strong. Their scouts are keeping tabs on each other consistently and they're gravitating towards one another. Gaius knows he can't take Upia if that means Nektos will be at his back so he must fight Nektos.

But he has to be ready for Nektos. Gaius remembers very well the Battle of Teutoberg Wald and he knows full well what'll happen if his men are hit while marching in a column. He also remembers Crassus and how he did his best to get his men out of marching order and into battle arrangements, but it still fell on its ass, and Gaius is not a huge fan of drinking gold. So he has scouts constantly keeping tabs on Nektos while he dances around keeping an eye out for terrain.

Eventually Nektos and Gaius get close enough to each other that its a simple brisk march to reach one another. This where both armies need to do their best to pick a battlefield. Sun Tzu stresses heavily how important it is to pick a battlefield for yourself.

What Gaius wants is a place with plenty of hills to help break up Nektos's hoplites, but that's a problem because that would hurt his cavalry, but it's still better than a perfect setting hoplite fight. Just as important, however, his reliance on foraging means he can't be that picky.

He hears Nektos is camping out near some hills, so he decides this isn't the ideal setting, but given that he's not going to be able to sustain this dance for much longer, he needs to take what he can get. So he rallies his men and begins the march, once again, scouts forward and in front.

Nektos's scouts notice that Gaius is marching at him. He can't stay bedded, he can't march, because doing so means that his opponent will overrun him when he's not in battle formation. He doesn't mind the terrain he's on, so rather than risk getting caught by a night time forced march, he gets his army ready, draws up battle plans, and prepares for the coming battle, scouts constantly letting either army know where each other are.

Finally Nektos and Gaius meet on the field of battle, dictated by the circumstances of their mutual objectives. They fight, Gaius beats Nektos, but sustains heavy losses, and realizes that although Nektos's army ran from the battlefield, it was too even, and there is no hope of taking Upia with the army he has. Therefore, he turns and leaves, marching out of Syria and hoping the Senate isn't too angry at him as he replenishes his army.

This is a very basic example of all the things that go into play when two armies decide they're going to fight each other. It's not a case of "MEET ME AT AGINCOURT AT DAWN, YOU FRENCH SWINE!", the circumstances dictated that both kings were going to fight there, because Henry V couldn't run any further, and this was the best location he could hope to fight the French on.

Many times, you'll see commanders simply say fuck it, I don't want to fight you there, and walk off, shielding their backs. The worst generals are the ones that see an opponent stop and just decide well fuck it, lets fight.

Consul Fabius had learned from Cannae and Lake Tresamene that you probably don't want to just attack Hannibal when Hannibal wants to be attacked. So he danced around and avoided battle, despite Hannibal desperately trying to goad him into one.

Then Scipio marched on Spain and cut off the Carthaginian land route to Rome. This is a great example of "you don't have to fight if you don't want to...unless you REALLY have to".

It was always dangerous, as mentioned in the Upia scenario, to march Around your enemy to your objective, because once again the opposing general had a decent idea of where you are, and marching does not involve walking in battle formation. It's usually long, battle-inefficient columns and parades where a lot of people may not even have the proper access to the things they need to go start a fight with someone. (Could someone correct me, I don't believe Roman legionaries marched with their pila, but retrieved them before the battle).

So to summarize:

-Efficient commanders know where each other are

-Efficient commanders are trying to secure an objective that isn't necessarily the battle itself.

-Commanders therefore dance with one another until one picks a spot to plop down.

-Urgency of the objective dictates whether or not a fight happens.

TL;DR

C'mon read the Upia story, it's awesome.

Sources include a library on ancient and medieval warfare that I own, I could cite them all individually if you wish.

EDIT: One final thing to add. These concepts are pretty much universally true to warfare. War itself has changed enormously throughout human history, but the concepts of how to achieve victory never have. You always need supplies, a clear objective, and to secure that objective. Even modern wars are waged in the same way, it's the conditions and the norms of each time period that change.

So there are tons of specifics that people can add. Yes, there have been instances of two commanders meeting up and going "So you want to take this to that field over there and get this shit done?" but that doesn't change the basic concepts of how or why it happens. Each era's method of handling or reaching these generalities fascinates me.