Answer:
When the Spaniards decided to conquer Mexico, they appointed Hernan Cortes to lead the army.
Step-by-step explanation:
A comma separates an introductory phrase from the rest of the sentence. In this case the introductory phrase ends after Mexico. Imagine this question. When did they appoint Hernan Cortes to lead the army? The answer is when the Spaniards decided to conquer Mexico. The comma goes between the question and the answer.