Who were the sea peoples?
When the fleets of the Sea Peoples and the Egyptians clashed in the epic battle at Medinet Habu about 1178 BC., the two adversaries both relied on ships with a revolutionary feature: loose-footed rigging that allowed the vessel to tack into the wind and to maneuver even in poor wind conditions. Both ships also feature a crow’s nest, a perch at the very top of the mast in which a sailor could position himself and act as a lookout for approaching enemy ships.
The Sea Peoples are the Philistines, Shardana, Sikila, Lukka and the Danuna. The Sea Peoples were responsible not only for innovations in seafaring vessels, but also for some land-based technological improvements as well. In the 13th century B.C.E., a new architectural element appeared in Canaan—specifically at the coastal site of ancient Ugarit (Ras Shamra, in modern Syria). Claude F. A. Schaeffer, the French archaeologist who directed the excavations at Ugarit, first identified the new type of stone building block—a squared, dressed stone called an ashlar—attributing it to a new ethnic element that he called the Ashlar Builders. Sometimes the ashlars were decorated with a marginal draft—that is, a smoother edge, or margin, from which the center boss protruded. Authors Stieglitz and Raban note that the Sea Peoples, having come from the Aegean to Canaan, are the most logical carriers of this new style to the mainland.
Ramses III and the captive "Sea Peoples".

Madinat Habu - the temple of Rameses III

Son of Setnakhte
Who were the "Sea Peoples"; Why did the sea peoples invade Egypt?

The "Sea Peoples", who we are told of on reliefs at Medinet Habu and Karnak, as well as from the text of the Great Harris Papyrus (now in the British Museum), are said to be a "loose confederation" of people originating in the eastern Mediterranean. From their individual names, we believe that they may specifically have come from the Aegean and Asia Minor. However, regardless of their organization as a "loose confederation", they did manage to invade Egypt's northern coast and apparently mounted campaigns against the Egyptians on more than one occasion.

The Sea Peoples were probably part of a great migration of displaced people. The migration was most likely the result of widespread crop failures and famine. In fact, we learn from an inscription at Karnak that Merenptah had already sent grain to the starving Hittites [ancient Anatolian].

Reference:

Shaw, Ian, The Oxford History of Ancient Egypt, Oxford University Press, 2000
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Who were the sea peoples?
Why did the sea peoples invade Egypt?
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References:

The Greek Age of Bronze - Sea Peoples

The Origin of the Sea Peoples