Causes:
1. Cardinal Richelieu's Ambitions: Cardinal Richelieu's desire to consolidate his power and increase the authority of the French monarchy leads to various conflicts and intrigues throughout the novel.
2. D'Artagnan's Arrival in Paris: D'Artagnan's journey to Paris, driven by his ambition to become a musketeer, sets in motion the main events of the story.
3. The Queen's Affair: Queen Anne's secret affair with the Duke of Buckingham threatens the political stability of France and becomes a central source of conflict.
4. Rochefort's Pursuit: Cardinal Richelieu's agent, Rochefort, relentlessly pursues D'Artagnan and the musketeers, endangering their lives and missions.
Effects:
1. Formation of the Musketeers' Friendship: D'Artagnan's encounters with Athos, Porthos, and Aramis lead to their close friendship and their commitment to protect each other at all costs.
2. Siege of La Rochelle: Cardinal Richelieu's decision to lay siege to the Protestant stronghold of La Rochelle involves the musketeers and reveals their prowess in combat.
3. Discovery of the Queen's Affair: Milady de Winter's betrayal and capture uncover Queen Anne's secret connection with Buckingham, which threatens to cause diplomatic repercussions.
4. Milady's Revenge: Milady de Winter's relentless pursuit of vengeance against D'Artagnan and the musketeers leads to several dangerous encounters and challenges their skills.
5. Siege of Arras: Athos's personal vendetta against Rochefort and Milady culminates in the dramatic siege of Arras, where the musketeers face off against their enemies in a final conflict.
6. Resolution of Conflicts: By the end of the novel, most of the conflicts are resolved, with D'Artagnan proving his worth as a musketeer and Cardinal Richelieu maintaining his political dominance.
These causes and effects drive the plot of "The Three Musketeers," showcasing the characters' loyalty, bravery, and the intricate political machinations of 17th-century France.