The battle of Abydos (411 BC) was a second Athenian victory won in the Hellespont during 411 BC, and played a major part in securing Athens's food supplies from the Black Sea and in restoring morale after the disaster at Syracuse in 413 BC (Great Peloponnesian War).
This was the first battle to take place after Thucydides ends, and we thus have to rely on the accounts of Xenophon and Diodorus Siculus. Their accounts of the battle are similar in outline, but differ in some details. In both versions the battle begins when Dorieus, son of Diagoras, a Rhodian serving on the Peloponnesian side and commanding a fleet from Rhodes entered the Hellespont. He was spotted by the Athenians, and a chase developed. The Rhodians were forced to run for the shore, where they came under severe pressure. Mindarus, the Peloponnesian admiral in the Hellespont, saw this battle developed and sailed out with his main fleet. A major naval battle then developed between the Athenian and Peloponnesian fleets. This was a hard-fought battle until Alcibiades arrived with Athenian reinforcements. At this point the Peloponnesians made for the shore, where they were saved from a total disaster by their ally the Persian satrap Pharnabazus. The Athenians managed to capture a number of enemy ships and then withdraw having won a victory.
Both of our sources agree on the first moments of the battle. Dorieus and his fleet entered the Hellespont where they were spotted by Athenian lookouts. Xenophon gives Dorieus fourteen ships, Diodorus doesn't mention the size of this fleet. We now come to our first disagreement. Both sources agree that the Athenians put to sea to intercept Dorieus, but disagree on the number of ships involved at this point. Diodorus gives a figure of seventy four ships, the entire Athenian fleet. Xenophon puts the figure at twenty, suggesting that only part of the Athenian fleet was involved at this stage. Dorieus responded by making for the shore, landing at either Dardanus or Rhoeteum. Both of these places were on the southern shore of the Hellespont, and were south-west of the main Peloponnesian base at Abydos.
The Athenians followed Dorieus and attempted to capture his beached ships, but apparently without success. According to Xenophon they eventually gave up and returned to their own base. In both accounts the Peloponnesian admiral Mindarus saw the fighting, and put to sea with his main fleet. Diodorus gives him eighty-four ships in his own fleet and a total of ninety seven once the two fleets were united, suggesting that Dorieus now had thirteen ships. Mindarus took command of the right wing, and his Syracusan allies commanded on the left.
The Athenians responded by bringing their entire fleet to face the new threat. Thrasybulus led their right wing, facing Mindarus, and Thrasyllus the left, facing the Syracusans. Both of our sources agree that the resulting battle was a hard fought and lengthy affair, lasting from early morning to mid-afternoon according to Xenophon. It was decided by chance. Alcibiades, now back in Athenian service, arrived in the Hellespont with either 18 or 20 ships. At first neither side knew for certain whose side the new arrivals would join, but it soon became clear that they were Athenians. The Peloponnesians reacted by attempting to escape to safety back at Abydus, where they would be protected by the Persian army of Pharnabazus.
According to Diodorus the Athenians captured ten ships during the pursuit, but a storm prevented them from pressing the pursuit. By the time they caught up with the Peloponnesians, they had run ashore and joined up with the Persians, and despite their best efforts the Athenians were unable to capture any more ships.
Xenophon doesn't mention the storm, but he does agree that fighting took place on the shore, between the Athenians attempting to capture ships, and the Peloponnesians and Persians. In this account the Athenians are more successful, capturing thirty enemy ships without their crews (presumably by towing them off the beach while their crews were onshore). They also recaptured their own ships lost during the main hard-fought naval battle.
In either case the battle ended as an Athenian victory, and combined with the earlier victory at Cynossema (411 BC) helped to prevent the Peloponnesians cutting of Athens's crucial supply lines to the Black Sea, where much of the city's food came from.