The battle of Delium (424 BC) was a costly Athenian defeat that came during an unsuccessful attempt to seize control of Boeotia (Great Peloponnesian War). In the summer before the battle the Athenian general Demosthenes had been in contact with some potential Boeotian rebels who were opposed to the policy of the Boeotian League (led by Thebes). The plan was for the rebels to seize Siphae, on the southern coast of Boeotian (the northern shore of the Corinthian Gulf) and Chaeronea, in the west of the area, and hand them over to the Athenians. At the same time the Athenians were to capture Delium, on the eastern edge of Boeotian (and the site of a temple to Apollo). The rebels hoped that this would trigger democratic revolts across Boeotia, and that the newly democratic cities would then support Athens.
The plan went wrong almost from the start. The Spartans discovered the plot and informed the Boeotians, so the element of surprise was lost. The Athenians then failed to properly synchronise their attacks. Demosthenes moved first, but when his fleet reached Siphae he found the place occupied by a strong Boeotian army. The Athenians were unable to make any progress, and the rebels decided not to act. Demosthenes was forced to retire without achieving anything.
The Athenian army, under the command of Hippocrates, only appeared on the scene after Demosthenes had retired. The temple at Delium was captured, and the Athenians began to work on fortifying the site. The short distances involved in some Greek warfare is well demonstrated here - the Athenians reached Delium on the third day after leaving Athens. They then spent the third and fourth days and most of the fifth day building the fortifications, before Hippocrates made a rather odd decision. The fortification work was completed by the afternoon of the fifth day. Instead of staying in the fortifications overnight, Hippocrates decided to begin the march back to Athens. His army consisted of 7,000 Athenian hoplites, some cavalry and a large force of light troops, mainly made up of resident foreigners and poorly equipped Athenian citizens. After marching for just over one mile the hoplites decided to pause and rest, but the light troops continued onwards. The Athenians would soon be forced to fight without them.
The failure of Demosthenes's naval expedition meant that the Boeotians had been able to concentrate on Hippocrates. By the fifth day of the expedition the Boeotians had gathered at Tanagra, close to Delium. They also had 7,000 hoplites, supported by 1,000 cavalry, 500 peltasts and 10,000 light troops.
At this date the Boeotian army was commanded by eleven generals, two from Thebes and nine from the other major members of the League. When they discovered that the Athenians were heading home ten of generals wanted to avoid battle, but the eleventh, Pagondas, son of Aeolidas, one of the two Theban generals, convinced his colleagues to offer battle. The Boeotian army then advanced towards the Athenians, before forming up on the far side of a hill. The Boeotians deployed in a somewhat unusual formation. Their basic formation was conventional, with the hoplites in the centre and the cavalry and light troops on the wings, but the Theban contingent, on the right of the line of hoplites, took up an unusually deep formation - twenty five men deep.
The Athenian deployment was more conventional. Once again their hoplites were in the centre and their cavalry on the wings, but their line was eight ranks deep. The main part of the battle only involved the hoplites, as the light troops and cavalry were initially held up by watercourse.
The Boeotians began to advance while Hippocrates was still moving along the Athenian line giving his pre-battle speech. He was forced to abandon his efforts when he was only half way along the line. Both sides then advanced towards each other at the run, and a stubborn clash between the two lines of hoplites began.
At first the Athenians were victorious on their right and in the centre, inflicting heavy casualties on some of the Boeotian contingents, and in particular on the Thespians. Pagondas responded to the crisis on his left by sending some of his cavalry from his right to his left, around the back of a hill just behind the battlefield.
On the Boeotian right the deep Theban formation was having more success, pushing the Athenians slowly back. Meanwhile, the cavalry had made its way round to the left, and now appeared on the Athenian's right flank. Believing that the cavalry was the first part of a fresh army the Athenian right panicked and fled. The panic spread along the line and the Athenian left also broke. The Athenian army scattered, with some men making for Delium, while others fled towards the mountains or the coast. The Boeotians mounted a pursuit, but the battle had been fought late in the day, and nightfall saved the Athenians from a worse disaster.
The battle was followed by some unusually drawn out negotiations between the two sides. In most cases a truce was quickly agreed to allow both armies to retrieve their dead, but in this case the two sides argued over the rights and wrongs of the Athenian invasion and of their occupation of Delium. Only after the temple had been recaptured (using an early flame thrower) did the Boeotians agree to let the Athenians recover the dead.
The battle had been a costly affair. The Athenians had lost nearly 1,000 men, most of them citizen hoplites, and amongst them Hippocrates. The Boeotians lost around 500 men. The Athenian casualties were amongst the highest suffered in any hoplite battle.