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Why did Ptolemy I Soter fight Perdiccas?

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Coin depicting Ptolemy I Soter — Ptolemy I Soter Ptolemy I Soter, portrait on a silver tetradrachm; in the British Museum

Ptolemy I Soter fought Perdiccas because he was part of a coalition of diadochoi (successors) who resisted Perdiccas’s growing authority and his ambition to rule the entire Macedonian empire as king.12

Following the death of Alexander the Great, Perdiccas served as regent and "supreme general," a position that caused friction with regional governors like Ptolemy, who believed the empire could not remain unified and sought to secure their own territories. In 321 BCE, Perdiccas marched from Asia Minor to invade Egypt and forcibly remove Ptolemy from power.123

Ptolemy successfully defended his position, aided by the strategic advantage of the Nile and the fact that Perdiccas’s troops were not fully committed to the campaign. The conflict ended when Perdiccas was murdered by his own mutinous officers after failing to cross the Nile.31