How did the French persuade the Spanish to join the American War of Independence in 1779?

The choice regarding the French persuading the Spanish to join the American War of Independence accurately highlights the strategic interests of both nations at the time. In 1779, the French approach was primarily focused on geopolitical advantages, and supporting the Spanish in their ambition to conquer territories such as Minorca and Gibraltar played a crucial role in securing Spain's involvement in the conflict against Britain.

The allure of territorial gains provided a compelling incentive for Spain to enter the war alongside France and the American colonies. Both France and Spain were motivated by a desire to weaken British power and influence, and the promise of conquest and expansion aligned closely with Spain's strategic goals.

The other choices, while they reflect various elements of international relations and warfare, do not capture the specific motivations that led to Spain's active involvement spurred by France. The notion of forming a military alliance against Britain indeed emphasizes a collaborative strategy, but the specific assurances regarding territorial expansion were key in gaining Spain's support in the broader context of the war.