When I recently reviewed Great Battles of the Classical Greek World, I was struck by the author's contradictions about the Battle of Delium. Therefore, I started to do more reading. As identified by the author, the primary sources are Thucydides and Diodorus, both available online.
In 424 BC, during the course of the Peloponnesian War, the Athenians were persuaded by political dissidents that there was a chance for 'regime change' in neighboring Boeotia. If only the Athenians would make a show of force, the dissidents would revolt against the conservative oligarchs and bring democracy to their city-states, eliminating a powerful Spartan ally.
Consequently, the generals Demosthenes and Hippocrates planned to execute a simultaneous, two-pronged attack: Demosthenes would sail west to unite with allies and take two cities by betrayal; Hippocrates would march north, seize Delium, fortify it, and prepare to defend it. Demosthenes had previously won fame for his victory at Sphacteria, but seems to have been junior to Hippocrates (who sends Demosthenes off on his voyage). The general, sometimes known as Hippocrates of Athens, is not the same person as the earlier Hippocrates, Father of Medicine.
On the map, Delium is at the middle-top; Athens is off map to the bottom in Attica; the major Boeotian city of Thebes is off map to the left. The god Apollo was believed to have been born on the island of Delos, and the island became increasingly sacred to the ancient Greeks; the separate city of Delium was named in honor of Delos because there was also a prestigious temple to Apollo there. It's 'mother' city was Tanagra. (Delium is today known as Dilessi/Dhilissi, Greece.)
Why Delium? It was conveniently located on the border of Boeotia, so it was considered a strategic location for future operations. It was on the coast, so the Athenian fleet could provide support (Boeotia was not a maritime power). Possession of its temple was probably thought to convey advantages to its conqueror. We're told the temple was prestigious at the time, though today it is little known. On the other hand, taking Delium put the Athenians on the far side of the Asopus River, in an exposed position.
Unfortunately for the Athenians, their two generals failed to coordinate properly. Demosthenes struck first, and betrayed by treachery and without Hippocrates as a diversion, failed to execute his mission. And the revolts failed to materialize.
The ancient sources do not tell us if or when Hippocrates learned of the failure of Demosthenes' mission. He acts as if he was not aware.
Hippocrates followed the plan. Even though he was too late to coordinate with Demosthenes, his departure from Athens was hurried and on short notice, and his forces were unprepared. On the third day after leaving Athens, Hippocrates seized the sanctuary of Apollo at Delium, and began to fortify the place by digging a trench, throwing up a rampart, and erecting wooden towers.
Why seize the temple of Apollo, and not the city of Delium itself? (The actual city of Delium seems to have been separate from the temple precincts.) The Boeotians considered the act to be sacrilege; the Athenians seem to have thought that by doing so, they were making the temple "their own." Perhaps the Athenians felt that possessing the temple would give them an advantage in battle. It seems startling that the Athenians would risk the displeasure of Apollo by digging on the consecrated ground and drinking from the sacred waters, but they argued that their actions were required by the exigencies of war. The taking of Delium may have occurred close to Apellaios, the time of Apollo's rites in late fall.
Meanwhile, the Boeotians were aware of the invasion – either from their scouts, or probably the plan had leaked – and were already gathering at Tanagra (5 miles from Delium). Without Demosthenes or the rebellions as a distraction, the Boeotians were able to assemble a large force.
On the fifth day after leaving Athens, at dinnertime, Hippocrates ordered the majority of his army to return to Athens, while he remained with a garrison to complete the fortifying of Delium.
Why did Hippocrates order his army away? Remember that the original plan predicted that no battle at Delium would be necessary, because the Boeotians would be tied up by uprisings and Demosthenes' attack; we don't know if Hippocrates knew at this point that Demosthenes had failed. So perhaps Hippocrates had always planned to leave Delium after a few days. Or alternately, perhaps Hippocrates' scouts and spies had informed him about the enemy mustering in strength at Tanagra, and so he ordered the withdrawal. After all, the Boeotians in Tanagra were between him and home, and there was a river to cross.
How much of a garrison was originally left at Delium? One would expect that a fortress must have been manned by infantry, but the sources only specifically mention cavalry.
Why did Hippocrates not accompany his army? Perhaps he thought they were escaping to safety, or perhaps he felt there was more likely to be a battle at Delium. Perhaps the heavy cavalry camped to wait for Hippocrates?
Why at dinnertime? Possibly Hippocrates had received word that the Boeotians were gathering, but I think not – the departing army did not act as if they were in danger. Perhaps the timing allowed them to cross the river at low tide?
The withdrawing Athenian army split into two portions: the hoplites camped a mile and a half away from Delium – just far enough to put themselves out of Boeotian territory – while the light troops pressed onward for home.
Who were these light troops? We know that Hippocrates took with him from Athens "a levy in mass of the citizens, resident aliens, and foreigners." Hippocrates' troops were described as unprepared and unequipped, which must apply to the light troops as the hoplites seem to have fought well in the battle. We also know that Athens did not field light troops in this period, and that many of these troops were unarmed. These 'light troops' were probably used as laborers to build the fortifications, who were then escorted out of Boeotia when battle seemed imminent (rather than be an impediment to the Athenians).
Why did the hoplites camp so close to Delium? Note that even though they departed Delium at dinnertime, there was still enough time after they entered Boeotian territory for scouts to inform the Boeotians, for the Boeotians to confer, for the Boeotian army to march to battle, then to fight a battle late in the day, and still have enough sunlight (on an early winter day) to begin the pursuit of the routed. Possibly the hoplites felt safe outside of Boeotian territory. Perhaps the hoplites had only escorted the light troops to safety, and would have returned to Delium if circumstances had allowed (although our sources say they were on their way "home"). Perhaps they just wanted to have their delayed dinner?
Where was the Athenian camp? Our sources claim the Boeotians were hesitant to attack because the Athenians were no longer in Boeotian territory. That would seem to put the campsite in the territory of the city of Oropus. That would mean the heavy infantry was on the east bank of the Asopus River, the traditional border. Was there a bridge or ford? Did the Athenian fleet assist them in crossing? Or did they wait for low tide?
Note that Hippocrates chose not to send his army back to Athens by sea (the Athenian fleet was at hand), but by land. He apparently underestimated the danger posed by the Boeotians, or thought they would not attack outside of Boeotia.
The majority of the Boeotarchs (leaders) of the Boeotian army felt that there was no reason to attack the withdrawing Athenians, since they were no longer in Boeotian territory. However, Pagondas, their commander-in-chief, persuaded them of the necessity of attack to discourage future incursions.
Pagondas obviously rushed to the attack to intercept the Athenians before they moved further away. (From his pre-battle speech, we know he knew the Athenians had crossed the border, but we do not know if he knew they had camped. Possibly his scouts would not cross the border until the council of war had authorized it.) If the Boeotians rotated command on a daily basis, Pagondas may also have wanted to attack while he was still the commander-in-chief and could pursue his aggressive strategy.
What was Pagondas' plan? His scouts had at least reported the Athenians marching across the border. If Pagondas simply pursued them, it would entail a long chase, a river crossing, and the possibility of being trapped between the marching Athenians and any Delium garrison force. I think it is more likely that Pagondas would cross the Asopus River upstream and try to cut the Athenians off.
The camped Athenians apparently did not deploy scouts or patrols, as the Boeotians were able to approach without being detected by them. However, Hippocrates learned of the approach of the enemy (possibly from the cavalry with him at Delium), and he apparently discerned that the attack would be on the camped troops and not Delium itself.
How did Hippocrates know the Boeotians meant to attack the withdrawing Athenians, and not the fortress at Delium? I think his scouts must have told him that the Boeotians were crossing the Asopus, moving away from Delium.
Hippocrates sent orders for the camped troops to deploy into battle line, and rushed to join his army.
Was the battle fought at the campsite? The Athenians were said to have camped outside of Boeotian territory. However, when Hippocrates speaks to his men before the battle, he says that they are in Boeotia – it seems likely to me that Hippocrates ordered his heavy troops to return across the Asopus River. Remember that this is known as the Battle of Delium, not the Battle of Oropus! (A contrary theory is that Hippocrates, lacking for time, used a previously written oration intended for a battle at Delium.)
What was Hippocrates' plan? My guess is that he wanted to unite his forces, now that he apparently knew the Boeotians had massed their forces. Therefore, he ordered the encamped troops to return across the Asopus River and form line of battle; meanwhile, he marched from Delium with most of the garrison, leaving only a small cavalry force behind. The garrison must have included infantry, but we know there was only cavalry left there before the battle – so it seems obvious Hippocrates brought the infantry with him. Similarly, we know the Athenians had cavalry at the battle, but no cavalry is mentioned at the encampment – so most likely, Hippocrates brought the cavalry with him also.
Did Hippocrates choose his battlefield? We know the battle was fought between two waterways, preventing both armies from fully deploying – just what Hippocrates might have wanted, if he knew the Boeotians outnumbered him. Or perhaps he simply marched to block the Boeotian line of march from Tanagra.
Did Hippocrates anchor his left flank on the Asopus River? It would seem an obvious move, and would explain what one of the waterways was.
It is interesting that, after all of the effort to fortify Delium, Hippocrates chose to give battle rather than fall back there.
The Boeotians advanced on the Athenians, deploying out of their vision behind an intervening hill. They sent a blocking force of cavalry to deter the cavalry left at Delium.
The Boeotians probably had not planned to fight a battle that day, but simply to pursue as far as possible before the light failed. However, once their scouts told them that the Athenians were recrossing the Asopus River, it seems likely the Boeotians would have changed course and followed the west bank of the Asopus toward the coast and directly into battle.
Note that Pagondas must have been aware that Hippocrates had marched most of the garrison out of Delium, as he only sent a screening force against Delium.
Why would Pagondas deploy behind a hill, then march his army over the crest? It would seem to delay or tire his army, precisely when time was critical due to the time of day. Was it simply for the element of surprise, and if so, did it 'wrongfoot' the Athenians? Or did he simply need to sort out his army after their march from Tanagra? Or did this give him greater room to deploy, compared to the road or trail from Tanagra which apparently was between a hill and a waterway?