In very militant Irish republicanism, Irishmen who served in the British Army during World War I are traditionally considered to have been traitors. Also, Behan is known to have always had a deeply troubled relationship with the Roman Catholic Church in Ireland. Even so, Behan is also known to have deeply admired Father William Doyle, a Dublin-born priest of the Society of Jesus, who served as military chaplain to the Royal Dublin Fusiliers as they fought in the trenches of the Western Front. Father Doyle was ultimately killed in action while running to the aid of wounded soldiers from his Regiment during the Battle of Passchendaele in 1917. Behan expressed his affection for Father Doyle in the memoir "Borstal Boy". Also, Alfred O'Rahilly's 1920 biography of the fallen chaplain is known to have been one of Behan's favorite books.