The year 1765 was a crucial year for both Lagrange and d'Alembert, but in very different ways. The two scientists had begun their correspondence in 1758, they grew closer after meeting in Paris in 1763. D'Alembert played an important role in Lagrange's decision to leave Turin in August 1765 for Berlin. In this article, we follow the course of the year 1765 through the letters exchanged by Lagrange and d'Alembert, as well as in their relations with notable personages of the day, including Frederick II, Clairaut, Euler, Daniel Bernoulli, Condorcet, Cesare Beccaria, Frisi and Caracciolo, which will help us understand more about the personality of Lagrange and the reasons behind his departure for Berlin.