Archidamus III, king of Sparta (r.359-338 BC) was a major Spartan leader during the period of the Theban Hegemony, and was unable to prevent a major decline in Spartan power.
Archidamus was the son and heir of Agesilaus II, a very able leader, but comparatively elderly by the 360s. One of his earliest appearances came in 378 in the aftermath of a failed Spartan surprise attack on Athens. The commander responsible for that attack was put on trial in Sparta, but acquitted, and some sources say that one reason for this was that his son was Archidamus's lover.
Archidamus was the heir presumptive to one of the two Spartan thrones, and played a major part in the defence of the cities interests in the Peloponnese in the aftermath of the major Theban victory at Leuctra in 371. In the aftermath of the battle the authorities at Sparta raised as many men as possible and sent them north under Archidamus. He was unable to alter the outcome of the campaign, but he was able to rescue the Spartan survivors and escort them back home.
The battle of Leuctra exposed the Peloponnese to invasion, and over the next decade Epaminondas led four campaigns in the Peloponnese. Early in these campaigns he liberated Messenia, reducing the number of helots available to Sparta. He also helped create a new Arcadian League, in the northern Peloponnese, a second attempt to weaken Sparta.
In 368 Archidamus commanded an Allied army that operated in Arcadia. This force included a force of mercenaries sent by Dionysius from Sicily, amongst them a group of Celtic mercenaries. This army captured Caryae and Parrhasia before an Arcadian and Argive army arrived to stop him. Archidamus withdrew into the hills, but at this point his Sicilian allies decided it was time to leave. They attempted to march away to the south, but ran into a force of Messenians. Archidamus had to rescue them and this exposed him to attack by the Arcadians and Argives. The resulting battle, fought near Balea, became known as the 'Tearless Battle' in Arcadia, because of the low number of Spartan casualties. Although this victory lifted Spartan morale, it had little long term impact. The Arcadians responded by founding the new city of Megalopolis, a major fortified position that further restricted Spartan power.
In 365 or 364 Archidamus captured Cromnus in Arcadia, a key position on the road between Sparta and their allies at Elean. The Arcadians responded by besieging the city. Archidamus led a relief army into Arcadia and attempted to force the Arcadians to lift the siege by ravaging their countryside. When this failed he attempted to part of their siege lines that ran over a hill, his troops ran into an elite Arcadia unit and were repulsed. This battle ended with a temporary truce, but only after Archidamus had been wounded. Eventually part of the garrison managed to escape, but more than 100 Spartan and allied troops were captured.
In 362 he defended Sparta against Epaminondas's last campaign in the Peloponnese. He helped defend Sparta herself against a surprise attack. The Theban and their allies won the main battle in this campaign, at Mantinea, but Epaminondas was killed in the battle and the heart went out of the Theban war effort.
Archidamus came to the throne in 359. He supported the Phocians during the Third Sacred War (355-346 BC), but was unable to prevent Philip II of Macedonia defeating the Phocians. In the last year of the war he led an army into Phocia, but withdrew after Philip II arrived in the area.
In 352 he commanded the Spartan forces during a war against Megalopolis, which again ended in failure.
He spent much of his time in the years after Mantinea serving as a mercenary commander in order to raise money to pay for mercenaries to fight Sparta's wars.
Archidamus was killed in 338 BC while campaigning in southern Italy. He was fighting on behalf of the Greeks of Tarantum against some of their Italian neighbours, the Lucanians, and was killed during fighting at Manduria.