Gallipoli

In Western Australia, May 1915, Archie Hamilton (Mark Lee), an 18-year-old stockman and prize-winning sprinter, longs to enlist in the Australian Imperial Force. He is trained by his uncle Jack (Bill Kerr) and idolises Harry Lascelles, the world champion over 100 yards. Archie wins a race with a bullying farmhand, Les McCann (Harold Hopkins), Archie running bare-foot and Les riding his horse bareback.
Frank Dunne (Mel Gibson) is an unemployed ex-railway labourer who has run out of money. He's an accomplished sprinter and hopes to win the prize money at the athletics carnival, he also bets a lot of money on himself winning. Archie and Uncle Jack journey to the athletics carnival. Frank is surprised when Archie defeats him and is bitter at first and he feels robbed of his bet. Eventually Frank approaches Archie in a cafe after getting over his loss and they both decide to travel to Perth to enlist. Before leaving, Archie gives all the prize money he won at the race to Jack and tells him that he will not be coming home for he had decided to enlist.
As Archie and Frank are penniless, they secretly hop on a freight train, then walk across the desert and stop for the night at a cattle station. Upon arriving in Perth, they arrange to stay with Frank's father, an Irish immigrant. Due to Frank's Irish heritage and general cynicism, he has little desire to fight for the British Empire. However, Archie persuades him to try to enlist in the Light Horse. Failing to ride a horse, Frank enlists in the infantry with three co-workers from the railway: Bill (Robert Grubb), Barney (Tim McKenzie), and Snowy (David Argue). Many of the motivations for enlistment are revealed: wartime ultra-nationalism, anti-German propaganda, a sense of adventure and the attraction of the uniform. All soldiers embark on a transport ship bound for Cairo. Frank and Archie are separated and embark on different troopships.
Some months later, Frank and his fellow soldiers train near the Pyramids and spend their free time in Cairo, drinking and visiting brothels. During a training exercise, Frank and Archie meet once again; Frank is able to transfer to the Light Horse, as they are now being sent to the Gallipoli peninsula as infantrymen.
The soldiers arrive at Anzac Cove and endure several days of hardships and boredom in the trenches. Frank's infantry friends fight in the Battle of Lone Pine on the 6 August. Afterwards, a traumatized Billy tells Frank what happened to the others: Barney was shot and killed, and Snowy is in a hospital, but in such bad condition that he is denied food and water. The following morning, Archie and Frank are ordered to take part in the charge at the Nek, a diversion in support of the British landing at Suvla Bay. Archie is ordered by Major Barton (Bill Hunter) to be the message runner. He declines the offer and recommends Frank for the role.
The Light Horse are to attack in three waves across a stretch of ground defended by Turkish machine gunners. The first wave is to go at 4:30 AM, after an artillery bombardment. Unfortunately, the commanders' watches are unsynchronized and the bombardment ends too early. The brigade's commander, Colonel Robinson, insists the ANZAC attack proceed; the first wave is cut down by the Turks within seconds. The second wave goes over, to a similar fate. Major Barton wants to halt the attack to end the carnage, but the Colonel says that somebody told him ANZAC marker flags were seen in the Turkish trenches, indicating that the attack was partially successful. The phone line goes dead. Barton gives Frank a message to carry to Brigade HQ but, when he arrives, the Colonel insists the attack continue.
Lieutenant Gray (Peter Ford), Major Barton's second-in-command, admits to Barton that he was the soldier who said that he saw marker flags, though he did not remember who told him. Frank suggests to the Major that he goes over the Colonel's head to General Gardner. Frank hurries to Gardner's headquarters down on the beach. The General is informed that, at Suvla, the British landing party is brewing tea on the beach. He tells Frank that he is reconsidering the attack. Frank sprints back to convey this news, but the phone lines are repaired and Colonel Robinson orders the attack to continue. Barton joins his men in the attack, climbs out of the trench, pistol in hand and signals his men to charge. Archie joins the last wave and goes over the top. Frank arrives seconds too late and lets out a scream of anguish and despair. As Archie's companions are cut down by gun fire he drops his rifle and runs as hard as he can. The final frame freezes on Archie being hit by bullets across his chest, head back, as if breaking the tape at the finish of a 100-yard sprint, and falling backwards (in an image evoking Robert Capa's The Falling Soldier).[citation needed]

Question: Why didn't Frank want to fight for the British?
Answer: Irish heritage and general cynicism