The 'Fighting Sixty-Ninth,' was an Irish regiment, although members of other ethnic groups served in it during the Civil War. It was called up briefly during the Spanish-American War, and in 1916, when it served on the Mexican border during General Pershing's expedition. When the United States entered World War I, the regiment was renumbered the 165th Infantry and assigned to be part of the new Rainbow (42nd) Division. Its members continued to refer to the regiment by its traditional name.

Duffy, a major and the senior chaplain of the 42nd Division, became an inspirational focus for the division and, later — for the whole American Expeditionary Force (AEF). The poet Joyce Kilmer, writing about the voyage of the division across the Atlantic, observed that every day there could be seen a line of soldiers, 'as long as the mess-line,' waiting their turn to have Duffy hear their confessions. Every morning, Kilmer (a war casualty himself) noted, a large crowd of soldiers would gather amidships on the transport where Chaplain Duffy would say Mass at an altar made from a long board resting on two nail kegs. The Division arrived in France in November 1917, and spent the winter training. In late February 1918, it took over front-line trenches from French forces at Luneville in the Lorraine sector. At dawn on March 20, Duffy and the men of the 42nd entered serious combat when a barrage of mustard gas shells burst among them. The bombardment lasted two days and there were over 400 casualties, the majority of them blinded.

The next few months were a horror. Duffy visited the front lines, heard confessions, said Mass and counseled the soldiers. It was by his 'ministry of presence' that made him influential and a legendary figure. When there was fighting, he would travel with a unit first-aid station, to provide physical and spiritual care to the wounded and the dying. He stayed near the heaviest fighting and exposed himself to constant danger as he moved from unit to unit. For his actions, he was decorated with the Distinguished Service Cross and the Distinguished Service Medal. The French awarded him the Croix de Guerre. General Douglas MacArthur had recommended that Duffy be given command of the Regiment and be promoted to Colonel. Duffy said later that 'there was little drinking while the regiment was in France, but what little there was did the men a lot of good but little harm.' The French Government made him a member of the Legion of Honor in 1920. After the war, he would tell war stories, but when he was praised for his effort he would say, 'Oh yes, I won the war,' and change the subject.

After the war, Duffy returned to New York City and eventually became pastor of the Holy Cross Church ('The actor's Church') at 237 W. 42nd St. In 1919, he published a best-selling book, 'The Story of the 165th Regiment,' chronicling his WWI experiences. Today he is perhaps best-known ,from the movie based on his life, 'The Fighting 69th,' made in 1940 which starred James Cagney and Patrick O'Brien as Father Duffy His funeral was at St. Patrick's Cathedral. He was buried at St. Raymond's Cemetery, the Bronx, while the Ninth Observation Squadron of 28 army planes circled in tribute overhead, and a rifle salute was fired at the grave. A large contingent of the veterans from the 'Fighting 69th' attended his funeral.

He sold his first songs to publishers in 1893. Cohan cast himself as an impertinent quipster with a heart of gold. Audiences loved him. He teamed up with Sam Harris, a gambler and boxing promoter, to form a producing team. His first Broadway hit (1904) was 'Little Johnny Jones,' in which he introduced his tunes 'Give My Regards To Broadway' and 'The Yankee Doodle Boy.' Cohan became an important Tin Pan Alley songwriter. Some of his hit songs include: 'You're a Grand Old Flag,' 'The Warmest Baby In The Bunch,' 'Life's A Funny Proposition After All,' 'I Want to Hear a Yankee Doodle Tune,' 'You Won't Do Any Business If You Haven't Got A Band,' 'Mary's a Grand Old Name,' 'The Small Town Gal,' 'I'm Mighty Glad I'm Living, That's All,' 'That Haunting Melody,' and 'Over There.'

ohan was an egomaniac and a philanthropist, particularly in regards to 'show folk.' The latter is ironic because of his virulent anti-union attitude. In the 1919 Actors Equity strike, Cohan pledged $100,000 to the anti-union Fidelity League. He later became president of it. Equity members called it 'Fido' for short. Actors Equity never forgave Cohan. He never signed up as an Actor's Equity producer, but was given a special exemption.

He wrote many hit plays: 'Forty-five Minutes from Broadway' (1905), 'The Talk of New York' (1907), 'Broadway Jones' (1912), 'The Song and Dance Man' (1923), 'American Born' (1925), 'Ah, Wilderness!' (1933), and 'I'd Rather Be Right' (1937). In 1925, he published his autobiography, 'Twenty Years on Broadway and the Years It Took to Get There.' In 1932, fearing his time had passed, he starred in his only musical film 'The Phantom President.' In 1942, the biographical musical film of Cohan's life, 'Yankee Doodle Dandy,' was released, with Jimmy Cagney starring as Cohan. It is how he is remembered today. Cohan died shortly after the film's release. He is buried in Woodlawn, Cemetery in the Bronx.