Egypt is difficult to enter (Strabo, Geography, 17.1.21): Invading Egypt - A Game Plan (7th – 4th centuries BCE)

Between the beginning of the seventh century BCE and the third century BCE Egypt was invaded about thirty times.
These invasions are documented in many sources of varied genres, origins, languages, and points of view. The present study is an attempt to evaluate the complexity of the war over Egypt in the discussed period.
Invading Egypt is a complex problem that challenged military leaders in ancient times. Nature had endowed Egypt with many defenses. Over the years, man-made defenses were added to the natural ones. In spite of these difficulties,
many leaders tried to invade Egypt. Some of them were tempted by the spoil of war; others were compelled to invade by Egypt’s own foreign policy that put their vital interests in danger.
In order to maximize their chances to succeed, prospective invaders had to put together an army larger and/or better trained and better equipped than that of the defender and to take into account all the obstacles both natural and man
made.
Once in Egypt the invader found himself (in most cases) in a war against the local ruler. The opposing sides in this war had entirely different objectives: the invader’s objective was a decisive victory over the defender. This includes: decisive victories in all field battles, the conquest of Memphis and, if needed, a pursuit after the Egyptian ruler southwards.
Failing to achieve even one of these goals meant the failure of the entire campaign. On the contrary, all the defender had to do was to prevent the invader from achieving at least one of his goals. Achieving this, the defender compelled
the invader to retreat. This dissimilarity dictated the tactics used by both sides: While the invader used (in most cases) brute force, the defender was inclined to use non-violent tactics as well.
During the period covered in this article one can observe, on the one hand, constant evolution in weapons, origin of the combatants and tactics, and on the other hand, abrupt changes that alter the balance of power between invaders and defenders. The evolution includes among others: The introduction of triremes, and later larger warships, the introduction of Greek and Carian mercenaries, and the use of elephants. The abrupt changes were the emergence of the Persian Empire and the upgrade of Egypt’s line of defense by Chabrias.