Calicut was a city located in the state of Kerala, India. During the Renaissance, a guild of the Assassin Order was based out of the city and received aid from Ezio Auditore da Firenze's Italian guild for several important assassination assignments.
The Assassin Guild in India was founded to keep an eye on the harbor and the rulers who had taken interest in India.
Manuel I of Portugal had also taken interest in India and he sent Pedro Álvares Cabral to Calicut with a fleet, which is where the Assassins Guild was based. It became clear that the Portuguese soldiers were not in Calicut for the spices, but to find and destroy the Assassins Guild. The Assassin started razing the Portuguese shops and storehouses. In the chaos that followed, they poisoned Cabral's most important soldiers, resulting in their death within minutes. Out of the thirteen ships that had traveled to Calicut, only four managed to sail back to Portugal. Through interrogation, the Assassins discovered that Manuel had indeed been after them and even knew the location of their Guild.
When he learned of the Assassins' actions, Manuel sent Vasco da Gama to India, with a fleet of 800 men. The Assassins tried to evacuate the city, but the citizens wouldn't listen. Upon Vasco da Gama's arrival he demanded the expulsion of all Muslims, knowing how much violence this would bring.
The Assassins called upon the citizens to obtain clothes and then planned an attack on the Portuguese captains. Knowing that stealth would not be effective in this situation, they attacked the guards and stabbed each one multiple times, sending a clear message to Portugal.