On Vasco da Gama's death the first sealed packet was found to contain the name of Dom Henrique de Menezes, who had won golden opinions as Pestana's successor at Goa. This young nobleman died at Cannanore on February 21st, 1526. The name contained in the next sealed packet was that of Pedro Mascarenhas, who was at this time Captain of Malacca. As he could not arrive for some months, the third packet was then opened, which contained the name of Lopo Vaz de Sam Paio, Captain of Cochin and a former officer of Albuquerque. Frequent complaints were sent to Portugal of the harshness and corruption of this Governor. It is asserted that he was incapable as well as cruel, and that the Portuguese fortresses were in a disgraceful state of neglect. He treated even the royal orders with contempt, and refused to hand over the government to Pedro Mascarenhas, whom he ordered into custody on his return from Malacca to claim his rights.
It was further made known to John III that Suláimán the Magnificent was setting on foot a great fleet for India. This was mainly due to the constant requests of the Venetians who were being ruined by the Portuguese monopoly, and was in general accordance with the policy of the greatest of the Ottoman rulers of Constantinople. The war between the Turks and Egyptians, which had allowed the Portuguese to develop in Asia, ended in 1517 with the overthrow of the Mameluke dynasty in Egypt. This great conquest of the Sultan Selim brought with it the submission of Syria and Arabia. Suláimán the Magnificent succeeded his father Selim in 1520, and began his reign by his famous campaigns in Hungary and against Rhodes. He was quite alive to the importance to Islám of checking the further advance of the Portuguese in the East, and the news that he was building a great fleet at Suez was perfectly true. It was placed under the command of Suláimán Pasha, and carried many Venetian and Christian adventurers as well as Turks and Egyptians.
Such being the dangers which threatened the Portuguese empire in Asia, John III selected to meet them the first really great successor to the office of Albuquerque, Nuno da Cunha. The new Governor was the eldest son of Tristão da Cunha, the navigator, and had had a large experience of Asiatic warfare. He was knighted by his relative, the great Albuquerque, in 1506, and had ever since been employed in voyages to the East and in hard-fought campaigns in Morocco. His chief feat of arms up to this time had been his conquest of Mombassa on the African coast in 1525, which he had followed up by exacting the tribute promised by the King of Ormuz to the Portuguese.
He left Lisbon in 1528 with a large fleet, carrying 4000 soldiers. He reached Goa in October, 1529, after a long voyage, and at once arrested Lopo Vaz de Sam Paio, and sent him back to Portugal in chains. His first measures were directed to the reform of internal abuses. With great activity he visited every Portuguese factory and fortress, punishing all evil-doers, and setting himself a noble example of personal probity. But he was not satisfied, like his predecessors, by merely securing old advantages and maintaining the former centres of trade. He devoted himself to opening up new provinces and developing the Portuguese commerce and dominion in other parts of India. The first Portuguese settlement on the Coromandel coast was at Saint Thomé near Madras, which received that name from the supposed discovery of the bones of St. Thomas the apostle of India. But Nuno da Cunha pushed farther up the coast and opened up a political connection with the wealthy province of Bengal.
Hitherto the Portuguese relations with Bengal had been purely commercial. In 1518 the first Portuguese ship, commanded by João da Silveira, reached Chittagong, and he there found João Coelho, who had arrived some months before from Malacca, having explored the eastern coast of the Bay of Bengal in a native craft. Silveira took a rich cargo on board, and after his visit it became an established custom for a Portuguese ship to visit Chittagong every year to purchase merchandise for Portugal. But Nuno da Cunha wished to do more than this, and to establish a regular factory and a political influence in the richest province of India.
An opportunity was afforded him in 1534, when the Muhammadan King of Bengal asked for the help of a Portuguese force against the Afghán invader, Sher Sháh. Nuno da Cunha promised his assistance, and at once sent a fleet of nine ships, carrying 400 Portuguese soldiers under the command of Martim Affonso de Mello Jusarte. The Portuguese contingent behaved gallantly, and its deeds are described in the first twelve chapters of the ninth Book of the fourth Decade of João de Barros, the contemporary Portuguese historian. Nuno da Cunha intended to follow in person, but he was prevented by the condition of affairs in Gujarát. It happened therefore that Portuguese authority was never directly established in Bengal. No royal factory or fortress was erected, and the Portuguese settlement at Húglí, where goods were collected for shipment to Portugal, was loosely considered to be subject to the Captain of Ceylon. The Portuguese in North-Eastern India remained to the end adventurers and merchants, and were never a ruling power.
The important events which prevented Nuno da Cunha from visiting Bengal were closely connected with the threatened approach of Suláimán the Magnificent's fleet from the Red Sea. It was well understood that that fleet would sail direct to the coast of Gujarát as the fleet of Emir Husain had done thirty years before. This knowledge made Nuno da Cunha very anxious to establish the Portuguese in a strong position on the coasts of North-Western India. Their main station in this neighbourhood had hitherto been the port of Chaul, where they had a factory and a small fortress. Portuguese agents were likewise established in the ports of Gujarát, but they were in no place masters of a strong defensive position.
To obtain a fitting site for a fortress in Gujarát was a principal aim of Nuno da Cunha's policy; not only for defence against the Muhammadans in India, but also as a bulwark against the expected Turkish fleet. Circumstances favoured him. The Mughal Emperor Humáyún was engaged in war with Bahádur Sháh, the King of Ahmadábád or Gujarát. In his extremity Bahádur Sháh sought to make an alliance with the Portuguese, and for this purpose he granted them the island of Bassein, which was then separated from the mainland by a narrow creek. Bassein lies about twenty-eight miles north of Bombay, and afterwards became the northern capital of Portuguese India, almost rivalling Goa in splendour and prosperity. At Bassein the Portuguese built a fort, but the place was not naturally defensible, and Nuno da Cunha set his heart on the possession of the rocky island of Diu, which had been one of the spots designed by Albuquerque for a Portuguese stronghold.
At last, in 1535, under the pressure of an invasion by Humáyún, Bahádur Sháh allowed the Portuguese to erect a fortress in Diu and to garrison it with their own troops. The fortress was rapidly and solidly built, and Bahádur Sháh and Nuno da Cunha signed a treaty of alliance. Such an alliance was not likely to last, and the murder of Bahádur Sháh in 1537, which took place on his return from visiting Nuno da Cunha on board his ship, caused a cry of treachery to be raised. It seems absolutely certain that the death of the King of Gujarát was due to a misunderstanding, but none the less friendship was owing to it replaced by bitter enmity. The fortress was not completed a moment too soon, for in 1538 the Turkish fleet, under Suláimán Pasha, after taking Aden by a stratagem, blockaded Diu by sea. Muhammad III, the nephew and successor of Bahádur Sháh, then besieged the place by land.