The siege of Halus (346 BC) was carried out as the same time as peace negotiations between Philip II of Macedon and Athens, and may have been part of Philip's wider plan for a campaign in central Greece (Third Sacred War).
The siege is only known to us through scattered references, mainly in the writings of Demosthenes. Halus was a minor polis in Phthiotis, the area around the head of the Malian Gulf, forty four around the gulf from the pass of Thermopylae. In 346 the Halians were involved in a dispute with Pharsalus, an inland town of southern Thessaly. The siege was conduced by Philip's general Parmenion, on behalf of Pharsalus.
The siege began early in 346. Our first mention of it comes when the Athenians were sending their first peace embassy to Philip II. The envoys moved to Oreus, then sailed across the sea to Halus. Using their diplomatic status they were able to pass through the Macedonian lines to meet Parmenion, and then continued on their way toward Macedon.
Later in the year the siege was still going on. A later Athenian embassy was delayed on its way back from Macedon, allegedly because Philip wanted them to help reconcile the Halians and the Pharsalians. However when the siege did finally end, Halus was handed over to the Pharsalians, and they exiled the inhabitants.
During the negotiations Philip insisted that Halus and Phocis were excluded from the peace terms. Demosthenes opposed this, while his rival Aeschines supported the proposal. Philip got his way.
It is possible that this siege was deliberately drawn out in order to give Philip an excuse to bring a large army down into Phthiotis, from where he was easy able to occupy Thermopylae, a move that effectively forced the Phocians to surrender to him, ending the Third Sacred War. If events had worked out slightly differently, this army might then have been used against Athens, although when the final conflict between the two did come (Fourth Sacred War), Philip actually bypassed Thermopylae, and instead crossed the mountains into Doris.