In order to understand and enjoy this novel to the fullest, one should be acquainted with a special kind of novel — the "swashbuckling novel," a novel which is filled to the brim with intrigue, adventure, and romance. One rarely, if ever, encounters this kind of novel in contemporary fiction, and it was very popular during the nineteenth century. Dumas was a master of this genre.
Basically, the swashbuckling novel combines the best elements of the novel of intrigue, the novel of adventure, and the novel of romance. The novel of intrigue involves plots and sub-plots in which one person or a group of people are involved in elaborate plots or schemes of one nature or another. This kind of novel is often, but need not be, about love and is frequently concerned with the intrigues of spies, the takeover of some enterprise, or political intrigue.
The novel of adventure is, as the term suggests, one which involves all kinds of adventures, most commonly those which take place on the highroads. For example, d'Artagnan's trip to London to retrieve the diamond tags for the queen and his various adventures and encounters with the enemy along the way constitute a novel of adventure. Usually, a main character's life is at stake, but this need not be necessarily so.
The novel of romance involves a simple love story of some nature, and there are several basic love stories in The Three Musketeers — for example, the duke of Buckingham's love for Anne of Austria, the queen of France; he will do anything for the pleasure of being in her presence. D'Artagnan is continually astonished at the duke's extravagant sacrifices — merely to please this lady. Likewise, d'Artagnan will undertake a dangerous journey solely because of his love for and devotion to Constance Bonacieux, a love that is, as we see toward the end of the novel, deeply reciprocated.
The term "swashbuckling" refers most often to a combination of the above three elements, accompanied by extreme histrionics — fantastic dueling and hair-raising escapades, narrow escapes, and desperate situations. These escapades are often seen as heroics — such as the episode where d'Artagnan and the three musketeers make a bet to stay in the bastion for an hour, and during this time, they stave off a number of the enemy.
Most often, the term "swashbuckling" is associated with dueling, especially when the hero is outnumbered by lesser swordsmen or when he encounters a superb opponent and yet easily disarms or conquers him. There is a good deal of swaggering (especially by Porthos); there is also a good amount of bantering, bragging, bravado, and exaggeration (by all three of the musketeers and d'Artagnan), and, of course, d'Artagnan is the perfect example of the swashbuckler because he is handsome, an expert dueler, and a superb swordsman. D'Artagnan is a young man captivated by love and romance and willing to undertake any type of adventure merely for the sake of adventure but certainly for the sake of the woman he loves.