The Portuguese conquest of Goa occurred when the governor Afonso de Albuquerque captured the city in 1510 from the Sultanate of Bijapur. Old Goa became the capital of Portuguese India, which included territories such as Fort Manuel of Cochin, Bom Bahia, Damaon, and Chaul. It was not among the places Albuquerque was supposed to conquer. He did so after he was offered the support and guidance of Timoji and his troops.
Albuquerque had been given orders by Manuel I of Portugal to capture Ormus, Aden, and Malacca only. Goa would remain under Portuguese control until 1961.
On 4 November 1509, Afonso de Albuquerque succeeded Dom Francisco de Almeida as Governor of the Portuguese State of India, after the arrival in India of Marshal of Portugal Dom Fernando Coutinho, sent by King Manuel to enforce the orderly succession of Albuquerque to office. Unlike Almeida, Albuquerque realized that the Portuguese could take a more active role in breaking Muslim supremacy in the Indian Ocean trade by taking control of three strategic chokepoints – Aden, Hormuz, and Malacca. Aden trade was controlled by Arabs, Hormuz by Persians, and Malacca by Muslim Malays. Albuquerque also understood the necessity of establishing a base of operations in lands directly controlled by the Portuguese crown and not just in territory granted by allied rulers such as Cochin and Cannanore.
Shortly after a failed attack on Calicut in January 1510, Albuquerque was replenishing his troops in Cochin and organizing an expedition with which to attack the Suez in the Red Sea, where the Mamluks were believed, correctly, to be preparing a new fleet to send to India against the Portuguese. Marshal of Portugal Dom Fernando Coutinho had been killed in Calicut, fortuitously leaving Albuquerque with full, uncontested command of Portuguese forces in India. The Portuguese force was composed of 23 ships, 1,200 Portuguese soldiers, 400 Portuguese sailors, 220 Malabarese auxiliaries from Cochin, and 3,000 "combat slaves" (Portuguese: escravos de peleja). The expedition set sail for the Red Sea in late January 1510, on 6 February anchored by Canannore, and on 13 February sighted the Mount of Eli.
By the Mount of Eli, Albuquerque summoned his captains to his flagship Flor de la Mar, where he revealed the objective of the expedition: He had orders from King Manuel I to subjugate Hormuz, but seeing as the Mamluks were assembling a fleet at Suez, he considered diverting from the original course of action to destroy it before it was ready.
Thereafter, the expedition resumed its course and anchored by the city of Honavar, where Albuquerque was approached by an acquaintance of the Portuguese: the powerful Malabarese privateer, Timoji (Thimayya). Timoji claimed to Albuquerque that it would be dangerous to leave for the Red Sea, as the Sultan of Bijapur Yusuf Adil Khan had been gathering within the nearby city of Goa the remnants of the Mamluk expedition destroyed in the Battle of Diu and refitting them with new ships to send against the Portuguese, likely in retaliation for the destruction of the city of Dabul by Almeida the previous year. The city however was scarcely defended as Yusuf had recently died and his heir Ismail Adil Shah was young and inexperienced. Knowing of the discontent among the Hindus of Goa after falling to the Muslim rulers of Bijapur in 1496, Timoji proposed to support Albuquerque in capturing the city. Timoji's timely proposition was not entirely coincidental, as Albuquerque had already received in Cochin envoys of Timoji requesting a rendezvous.
Upon assembling with his captains, Albuquerque convinced them that it was crucial that they attack Goa.
On 16 February, the Portuguese armada sailed into the deep waters of the Mandovi River. Supported by 2,000 men of Timoji, the Portuguese landed troops commanded by Dom António de Noronha and assaulted the fort of Pangim, defended by a Turkish mercenary Yusuf Gurgij and a force of 400 men. Yusuf was wounded and retreated to the city and the Portuguese captured the fort along with several iron artillery pieces. At Pangim, Albuquerque received envoys from the most important figures of Goa, and proposed religious freedom and lower taxes if they accepted Portuguese sovereignty. Thereafter they declared their full support for Portugal and Albuquerque formally occupied Goa on 17 February 1510, with no resistance.
Albuquerque reaffirmed that the city was not to be sacked and that the inhabitants were not to be harmed, under the penalty of death.
In the city, the Portuguese found over 100 horses belonging to the ruler of Bijapur, 25 elephants, and partially finished new ships, confirming Timoji's information about the enemy's preparations. For his assistance, he was nominated tanadar-mor (the chief tax-collector and representative) of the Hindus of Goa. The Muslims on their part were allowed to live by their laws under their own Muslim magistrate, Coje Bequi.
Expecting retaliation from the Sultan of Bijapur, Albuquerque began organizing the city's defences. The city's walls were repaired, the moat was expanded and filled with water, and storehouses for weapons and supplies were built. The ships were to be finished and pressed into Portuguese service, and the five fording points into the island – Banastarim, Naroá, Agaçaim, Passo Seco, and Daugim – were defended by Portuguese and Malabarese troops, supported by several artillery pieces.
At the same time, Albuquerque sent friar Luiz do Salvador ahead of an embassy to the court of the neighbouring Hindu Vijayanagara Empire, hoping to secure an alliance against Bijapur.
Ismail Adil Shah's counterattack
Unbeknownst to Albuquerque, Ismail Adil Shah had just agreed on a truce with the Vijayanagara Empire, and could divert many more troops into recapturing the city than expected. To that effect, he sent a Turkish general, Pulad Khan, with 40,000 troops, which included many experienced Persian and Turkic mercenaries, that defeated Timoji's troops on the mainland. Ismail then set up his royal tent by the Banastarim ford, waiting for the monsoon to trap the Portuguese before giving Pulad Khan the order to assault the island.
Albuquerque was informed of this plan through the Portuguese renegade João Machado, who was now a prestigious captain in Ismail's service, though he remained Christian. He was sent to convince his countrymen to surrender or flee. Trusting the strength of his defensive position, Albuquerque rejected Machado's propositions. Machado also told Albuquerque that the Muslims within the city kept Ismail informed of Portuguese numbers and movements.
With the coming of the monsoon rains however, the Portuguese situation became critical: the tropical weather claimed a great number of Portuguese lives, foodstuffs deteriorated, and the Portuguese were stretched too thin to hold back the Muslim army. Under these conditions, Pulad Khan launched a major assault on 11 May, across the Banastarim ford at low tide amidst a heavy storm, quickly overwhelming the small number of Portuguese troops. As defences crumbled, a Muslim revolt broke out in the outskirts of Goa, in spite of the agreement with Albuquerque, which he would remember in the future. The Portuguese hurriedly retreated into the city walls, with the aid of their Hindu allies, abandoning several artillery pieces by the riverside.
The following day, Pulad Khan ordered an assault against the city but was repelled. Only now did Albuquerque learn from friar Luiz of the truce between Bijapur and Vijayanagara, and he spent the rest of May preparing a retreat. Albuquerque refused to set fire to the city since this would announce their retreat to the besiegers and instead ordered a great quantity of spices and copper to be scattered on the streets to delay the enemy's advance. Before leaving however, he had Timoji with fifty of his men execute the Muslim inhabitants within the citadel, but also took several women that had belonged to Adil Khan's harem onto his ship, to later offer them as maids-in-waiting to Queen Maria. Before daybreak of 31 May, the remaining 500 Portuguese embarked under enemy fire, covered by a small number of Portuguese soldiers holding back the advance of enemy troops that breached the city walls. Ismail then solemnly retook possession of the city, to the sound of trumpets.