Why is it dangerous for the ship to land on the island of Elba in The Count of Monte Cristo by Dumas?
The island of Elba is the location of Napoleon Bonaparte's exile forced upon him as a result of his abdication at Fountainbleau, a treaty signed with the Allied Forces. While at Elba, Napoleon organized forces to aid in his return to France after he learned of the reestablishment of the monarchy in France with Louis XVIII of the House of Bourbon reclaiming the throne.
With this reestablishment of the Bourbon king and Napoleon's having been a key figure in the French Revolution which effected the guillotining of King Louis XVI and his quieen, Marie Antoinette, any association with Bonaparte would be viewed as an act of treason. Because of this condition, the innocent Dantes is implicated as being a Bonapartist and, thus, thrown into the prison at the Chateau d'If.
The real reason for his imprisonment is the fact that the Deputy Procureur, Monsieur de Villefort, wishes to marry into a wealthy royalist family as well as desiring to advance his political career. Doing this demands that he be perceived as a loyal follower of the monarchy.
So, when de Villefort learns that the letter which Dantes has delivered to Elba is from a Monsieur Noitier, he realizes that his career is jeopardized by his father, who is a stalwart Bonapartist involved in the plot to return Napoleon to France, he feels that he must remove Dantes from society so that nothing of this letter and trip to Elba will be learned by those so important to him in The Count of Monte Cristo.