Darius II (r.423-404 BC) Ochus was the Persian Emperor during the second half of the Great Peloponnesian War, and his money played a major part in the eventual Spartan victory. He was the son of Artaxerxes I and one of his concubines and was called Ochus before he came to the throne.
Darius came to the throne in 423, after a brief period of court intrigue. Artaxerxes I had died in 425 after a long reign, and was succeeded by his son Xerxes II. The new Emperor ruled for only 45 days before he was murdered by the son of one of his father's concubines (Secydianus or Sogdianus). Darius killed the murderer, and then took the throne as Darius II. Darius had served as satrap of Hyrcania before he came to the throne, so had some experience of political power. Even so his reign was dominated by palace intrigue, and he was said to have been dominated by his half-sister and wife Parysatis.
Darius came to the throne just before the end of the first phase of the Great Peloponnesian War between Athens and Sparta. In 421 the two sides agreed the Peace of Nicias, which largely restored the situation at the start of the war. For the next few years the two powers managed to keep to the terms of the treaty, although there were Athenian and Spartan troops on opposite sides at the battle of Mantinea (418 BC).
The second part of the war began when the Athenians decided to intervene on Sicily and support their allies on the island against Syracuse. A massive Athenian army landed on the island on 415, and in the following year began a siege of Syracuse (414-413). This ended in disaster, and the entire Athenian army was lost. The Spartans decided to help Syracuse, and the war officially resumed in 414.
Soon after the end of the Sicilian expedition the Persians decided to become involved in the war, on the side of Sparta. Presumably Darius II must have made this decision, but the actual intervention was left to the two satraps in the area - Tissaphernes of Sardis, who wanted the Spartans to operate in western Asia Minor and Pharnabazus of Hellespontine Phrygia, who wanted them to act in the Hellespont region. The two satraps often appear to have been more interested in their own rivalry than in the war in Greece. This allowed the Athenians to play the two off against each other, and helped extend the war. The problem was eventually solved when Cyrus the Younger, a royal prince, was given control of the province of Asia Minor.
In 412 the Spartans and the Persians agreed the Treaty of Miletus. The Persians agree to fund the Peloponnesian fleet while Sparta agreed to give them a free hand in Asia Minor. Although they were on the back foot the Athenians were able win a series of naval victories during this period, but Persian money kept the Spartans in the war. In 407 Darius made his younger son Cyrus the Younger commander-in-chief in Asia Minor, and with a Royal prince to control it the Persian war effort became more effective.
Finally, at the battle of Aegospotami (405 BC) the last Athenian fleet was destroyed. The city was besieged and finally surrendered in 404 BC. Darius II died in the same year, having survived long enough to see his intervention in the war finally bear fruit. However the last year of his rule was marred by a serious revolt in Egypt (405), which the Persians were unable to put down for several decades (the Persians didn't regain control of Egypt until 343 BC). He was succeeded by Artaxerxes II. Soon afterwards the new king had to defeat a revolt by his brother Cyrus the Younger, who recruited a force of Greek mercenaries to support his bid for the throne. Although Cyrus was defeated, the success of the Greeks in escaping from the heart of the Persian Empire is said to have encouraged Philip II of Macedon and Alexander the Great in their plans to invade the Empire.