The Battle of Diu sometimes referred as the Second Battle of Chaul was a naval battle fought on 3 February 1509 in the Arabian Sea, near the port of Diu, India, between the Portuguese Empire and a joint fleet of the Sultan of Gujarat, the Mamlûk Burji Sultanate of Egypt, the Zamorin of Calicut with support of Ottomans, the Republic of Venice and the Republic of Ragusa (Dubrovnik).
The Portuguese victory was critical: Mamluks and Arabs retreated, easing the Portuguese strategy of controlling the Indian Ocean to route trade down the Cape of Good Hope, circumventing the traditional spice route controlled by the Arabs and the Venetians through the Red Sea and Persian Gulf. After the battle, Portugal rapidly captured key ports in the Indian Ocean like Goa, Ceylon, Malacca and Ormuz, crippling the Mamluk Sultanate of Egypt and the Gujarat Sultanate, greatly assisting the growth of the Portuguese Empire and setting its trade dominance for almost a century, until it was taken during the Dutch-Portuguese Wars and the Battle of Swally won by the British East India Company in 1612. It marks the beginning of the European colonialism in Asia. It also marks the spillover of the Christian-Islamic power struggle, in and around the Mediterranean Sea and the Middle East, into the Indian Ocean which was the most important region for international trade at the time.
The Portuguese had eighteen ships commanded by the Viceroy, with about 1,500 Portuguese soldiers and 400 local combatants from Cochin. The Allied side had one hundred ships, but only twelve were major vessels; the rest were small shallow-draught craft. After detecting the Portuguese, who approached from Cochin to the north, and fearing their technical superiority, the Egyptians decided to take advantage of the port of Diu and its fort, which had its own artillery. It was therefore decided to stay anchored at the port and await an attack from the Portuguese. This may also have been due to the training of the Egyptians, who were used to the more sheltered bays in the Mediterranean. There they also relied upon land-based artillery reinforcements to defeat the enemy. The Portuguese started the battle with a massive naval bombardment using their on board artillery, followed by hand-to-hand combat in the harbour of Diu.
These Portuguese ships had guns of greater caliber, better artillery crews, and were better manned and better built. The Portuguese naval infantry also had an advantage over the Egyptians, not only because they were heavily armed and equipped (armour, arquebuses and a type of grenade made of clay with gunpowder inside), but also because they were seasoned professional seamen.
The tough state-of-the art multi-rigged Portuguese carracks and smaller fast caravels had been developed over the previous decades to cope with the storms of the Atlantic Ocean and were bristling with cannons. The smaller Indian Ocean dhows and Mediterranean-type galleys launched by the coalition of the Samoothiri Raja, Gujarat and Egypt were no match. The Portuguese ships were able to shoot their powerful cannons and thus dissuade the smaller craft from coming near them. Even when they did come near, the smaller galleys and dhows were low in the water, and so unable to board the Portuguese ships, while being sprayed from above with small arms, grenades and smaller caliber cannon.