How did Ptolemy I Soter keep control of Egypt during Perdiccas’s invasion?

Ptolemy I Soter successfully defended Egypt against the invasion led by Perdiccas in 321 BCE through a combination of strategic positioning and the exploitation of his opponent's military failures.

Ptolemy had previously secured a significant political advantage by diverting the corpse of Alexander the Great to Egypt, which bolstered his legitimacy. When Perdiccas marched from Asia Minor to seize control of the region, he was unable to successfully cross the Nile. This military stalemate severely undermined Perdiccas’s authority, leading to a mutiny among his own troops. Ultimately, Perdiccas was murdered by his officers, effectively ending the threat to Ptolemy’s rule.123