Demosthenes was also the name of an Athenian general during the second Peloponnesian War
Demosthenes was nowhere near as involved in the Athenian political arena as some of his fellow generals. This means the biggest source for him is Thucydides, this is unfortunate since Thucydides seems to have disliked Demosthenes and attributed much of his success to luck.
In 426B.C. he led an unsuccessful invasion of Aetolia, following this he succeeded in winning two brillant victories against a combined Ambraciot and Peloponnesian army.
His greatest achievement was the victory at Pylos in 425B.C., where he was not even a general. He went along with a fleet sailing to Corcyra, with the aim of taking and holding a key port in Spartan territory. He succeeded and he was left there with a small force, he mobilised the force to fortify the position. Outnumbered he organised his troops and called for the fleet to return and reinforce his position. He succeeded in holding the harbour against the larger Spartan force until the Athenian fleet returned. The most significant event of the Pylos campaign was the capture of 300 Spartans including 120 officers on the island of Sphacteria. This is traditionally attributed to Cleon, but the original work of surrounding the Spartans was done by Demosthenes. It is suggested by Aristophanes in his play Knights that Demosthenes resented Cleon taking the glory. The capture of the Spartans provided the Athenians with their most valuable bargaining chip from the Archidamian War.
The next year he succeeded in surprising Nisaea, however he failed to take Megara and in an attack on Boeotia he failed to land his men at Siphae, because the enemy had been warned. This failure meant he was not trusted with another major command for 11 years.
Demosthenes died at the end of the Sicilian Expedition in 413B.C., he was sent out to aid Nicias at the head of a relief force, however Nicias had already made too many critical errors by that time. After an attack on Epipolae failed, Demosthenes advised returning to Athens, however there was a lunar eclipse and Nicias being superstitious consulted a soothsayer who advised waiting for another month. This led to the disastrous battle in the harbour where many Athenians died and both Nicias and Demosthenes were captured. Gylippus the Spartan general and the man in charge of the Peloponnesian forces did not want them killed, however his Corinthian subordinate Gongalus and the Syracusans executed them anyway.
Demosthenes was an inventive general who tried to break the stalemate caused by the Periclean strategy, he was a skilled tactician and an inspired leader of men, his only failing was that his plans tended to be too involved and elaborate.