Conversation so far:
What can you tell me about The Vicomte of Bragelonne: Ten Years Later?
After 35 years of loyal service, D'Artagnan resigns as lieutenant of the Musketeers.
Why would that make D'Artagnan resign as lieutenant of the Musketeers?
D'Artagnan perceives the young king Louis XIV as weak-willed.
What happened next after d'Artagnan resigns as lieutenant of the Musketeers?
D'Artagnan resolves to aid the exiled Charles II.

Question: How does he help Charles?
Self-contained rewrite: How does d'Artagnan help Charles?