The battle of Pelusium (early 525 BC) was the decisive battle of the first Persian invasion of Egypt, and saw Cambyses II defeat Psamtik III, opening the rest of Egypt to conquest.
Cambyses came to the throne in 530, five years before the invasion. He put some effort into preparing for the invasion, gathering a major army, assembling his allies, and making an agreement with the local Arabs, who agreed to provide water during the crossing of the Sinai desert.
The Persians were aided by Polycrates, tyrant of Samos, who sent forty triremes. More importantly Phanes, one of the commanders of the Greek mercenaries in Egyptian employment, betrayed his master and sent vital information to Cambyses.
Psamtik posted his army near the Pelusian mouth of the Nile, and took up a defensive position. Both armies contained Greek troops - Cambyses raised his from his Ionian and Aeolian possessions, while the Egyptians employed mercenaries.
Herodotus records very little detail of the battle itself, describing it as fierce with heavy loses on both sides. He does provide colourful anecdotes from before and after the fighting.
Phanes had left his sons in Egypt when he fled to Cambyses. The Greek mercenaries in the Egyptian army decided to punish him for this by killing his children in front of the two armies, mixing their blood with wine and drinking the mixture.
Herodotus visited the battlefield about seventy-five years later, and reported that the bones of the dead were still lying in the desert. He claimed to have examined the skulls and found that the Persians had thin, brittle bones and the Egyptians thick solid bones. He suggested that this was because the Egyptians normally shaved their heads, and the sunlight thickened their bones. This might suggest that the battle took place on the edge of the desert, rather than on cultivated land, although it does seem a long time for the bodies to have remained visible and unburied.
The Persians went on to capture Heliopolis, and then besieged Memphis. According to Herodotus Psamtik was captured, and sent into exile in Susa, where he later committed suicide after being discovered plotting against the Persians.
Cambyses remained in Egypt for most of the rest of his reign. He planned an attack on Carthage, which never took place. An army that was sent to the Oasis of Amon at Siwah was lost in the desert. He led an expedition into Nubia which had some success.
In March 522 a revolt broke out in the heart of the empire, commanded by someone claiming to be Cambyses's brother Bardiya (Smerdis to the Greeks). Cambyses left Egypt, but died in Syria on the way home and was eventually succeeded by a distant relative, Darius I.