The Duellists

In Strasbourg in 1800, fervent Bonapartist and obsessive duellist Lieutenant Gabriel Feraud (Harvey Keitel) of the French 7th Hussars, nearly kills the nephew of the city's mayor in a sword duel. Under pressure from the mayor, Brigadier-General Treillard (Robert Stephens) sends a member of his staff, Lieutenant Armand d'Hubert (Keith Carradine) of the 3rd Hussars, to put Feraud under house arrest. As the arrest takes place in the house of Madame de Lionne (Jenny Runacre), a prominent local lady, Feraud takes it as a personal insult from d'Hubert. Matters are made worse when Feraud asks d'Hubert if he would "let them spit on Napoleon" and d'Hubert doesn't immediately reply. Upon reaching his quarters, Feraud challenges d'Hubert to a duel. The duel is inconclusive; d'Hubert slashes Feraud's forearm but is unable to finish him off, because he is attacked by Feraud's mistress. As a result of his part in the duel, d'Hubert is dismissed from the General's staff and returned to active duty with his unit.
The war interrupts the men's quarrel and they do not meet again until six months later in Augsburg in 1801. Feraud immediately challenges d'Hubert to another duel and seriously wounds him. Recovering, d'Hubert takes lessons from a fencing master and in the next duel (held in a cellar with heavy sabres), the two men fight each other to a bloody standstill. Soon afterwards, d'Hubert is relieved to learn he has been promoted to captain. Military discipline forbids officers of different ranks from duelling.
The action moves to 1806 when d'Hubert is serving in LÃ¼beck. He is shocked to hear that the 7th Hussars have arrived in the city and that Feraud is now also a captain. Aware that in two weeks time he is to be promoted to major, d'Hubert attempts to slip away but is spotted by Feraud's perpetual second. Feraud challenges him to another duel, which is to be fought on horseback with sabres. D'Hubert slashes his opponent across the forehead; Feraud, blinded because the cut bleeds heavily into his eyes, cannot continue the fight. D'Hubert considers himself the victor and leaves the field ebullient.
Soon afterwards, Feraud's regiment is posted to Spain. The pair chance upon each other, during the French Army's disastrous retreat from Moscow in 1812. Before they can resume the duel, Cossacks attack forcing d'Hubert and Feraud to fight together, rather than each other.
Two years later, after Napoleon's exile to Elba, d'Hubert is a brigadier-general recovering from a leg wound, at the home of his sister Leonie (Meg Wynn Owen) in Tours. She introduces him to Adele (Cristina Raines), niece of her neighbour (Alan Webb). The couple fall in love and are married. A Bonapartist agent (Edward Fox) attempts to recruit d'Hubert, as rumours of Napoleon's imminent return from exile abound. D'Hubert refuses to command a brigade if the Emperor returns from Elba. When Feraud, also a brigadier-general and a leading Bonapartist, hears this he declares d'Hubert is a traitor to the Emperor. He claims that he always suspected d'Hubert's loyalty, which is why he challenged him to a duel in the first place.
After Napoleon is defeated at Waterloo, d'Hubert joins the army of Louis XVIII. Feraud is arrested and is expected to be executed for his part in the Hundred Days. D'Hubert approaches the Minister of Police Joseph FouchÃ© (Albert Finney) and persuades him to release Feraud (without revealing d'Hubert's part in his reprieve). Feraud is paroled to live in a certain province under police supervision.
After Feraud learns of d'Hubert's promotion in the new French Army, he sends two former officers to seek out d'Hubert, so he can challenge him to a duel with pistols. Eventually the two men meet in a ruined chÃ¢teau on a wooded hill. Feraud rapidly discharges both his pistols, before being caught at point blank range by d'Hubert, who refuses to shoot him because tradition dictates he now owns Feraud's life. He tells Feraud he must submit to his decision, that in all future dealings Feraud shall conduct himself "as a dead man".
The duel ends and d'Hubert returns to his life and happy marriage, while Feraud returns to his provincial exile. The closing image of the film depicts Feraud in silent contemplation, gazing at the horizon in utter solitude unable to pursue the obsession that has consumed him for so many years.

Question: What is he unable to pursue ?
Answer: He is unable to pursue the obsession that has consumed him for so many years.