In October 1762, after several French defeats in North America and India during the Seven Year Wars, Louis XV accepted a plan from his ministers to invade the Portuguese colony of Rio de Janeiro. By then, Rio was the port from where all the gold from Brazil was being sent to Portugal (and from there to Britain, as Portuguese were buying almost all their manufactured products from the British). The idea of Louis XV was to score a last huge victory, conquering an important colonial territory that could give him a trump in the peace negotiations, or by recognizing his conquest or by trading it for other lost French territories.
The plans included to send a fleet formed by 22 ship (ten of them from the Royal Navy, other ten would be bought in Brest and two in Nantes). The commander of the fleet would be in charge of Beaussier de l’Isle. He would send to Rio 5150 soldiers, commanded by the Count D’Estaing, who would be nominated the French “Vice-King” in Brazil.
The French were well prepared they had several maps of Rio made by D’Estaing (who had been there in 1757), and the Portuguese couldn’t offer resistance, as the governor of Gomes Freire de Andrade was in South Brazil with the main part of his troops in order to fight the Spanish invasion of Rio Grande do Sul. Probably, if they French had really invaded Rio, they would have conquered it.
However, the French were not able to create the fleet, and exhausted by the war they signed the peace only four months after the approval of the plan. But, if we give them some POD that can allow the war to last longer without changing too much the outcome of it (BTW, which kind of POD could create this situation), and they conquer Rio before the peace, what would happen? Would they receive it? If they decide to exchange it for other territory, what could they gain? Could the British decide to destroy this trump, by invading Rio too in order to expel the French and avoid their control of the Brazilian gold? And what could they ask from the Portuguese in return for this “favour”?