Another of his primary duties as a chaplain was to bury the dead.
"I knew these men so well and loved them as if they were my younger brothers. It has been the saddest day of my life. Well, it is the last act of love I can do for them and for the folks at home," he said in his diary at the conclusion of one battle, according to the New York archdiocese archives' exhibit "The Great War and Catholic Memory."
When the division learned of the armistice two days late, on Nov. 13, 1918, the troops celebrated. But the priest recorded in his diary: "I could think of nothing except the fine lads who had come out with us to this war and who are not alive to enjoy the triumph. All day I had a lonely and aching heart."
Kilmer and Duffy served with another Catholic who would rise to prominence: William "Wild Bill" Donovan. Donovan would receive the Congressional Medal of Honor for his wartime service, and during the Second World War he would head the Office of Strategic Services, the U.S. covert agency that was predecessor to the Central Intelligence Agency.
The Fighting 69th became "the best-known face of Catholic soldiers in the U.S. Army," O'Brien said. "Although they were linked by an overarching Irish-American imagery, they also proudly included Catholics from other ethnic backgrounds and Jewish-Americans as well."
Its image became "iconic" due to the 1940 movie of the same name, which according to O'Brien had a "strong message of ethnic pluralism."
As for Duffy, his influence would continue. He was a ghost writer for New York Gov. Al Smith during controversy over whether the Democratic presidential candidate's Catholic faith would interfere with his ability to be president. Since 1937, five years after his death, his statue has stood in Duffy Square in the northern triangle of New York's Times Square. It shows a priest standing before a 17-foot tall Celtic Cross.
At a time when Catholic bishops avoided forming national organizations, it was during World War I when the U.S. bishops organized the unprecedented National Catholic War Council, a forerunner to the U.S. Conference of Catholic Bishops. In the 1920s the council, whose name had been changed to the National Catholic Welfare Council, proved an important platform for many programs and causes. It led the national response to an attempt to ban Catholic schools in Oregon backed by the anti-Catholic Ku Klux Klan and the Scottish Rite Masons.
This council also proved to be the seedbed for future leaders like Msgr. John A. Ryan, who was "strongly influenced by the social justice message of Rerum Novarum," the encyclical of Pope Leo XIII, O'Brien said. Ryan would play an important role in the Roosevelt administration and was a counterweight to the polemical populist "Radio Priest" Father Charles Coughlin, who turned towards anti-Semitism and fascism during the 1930s.
But during World War I, it was anti-German sentiment that ran high. Many German-Americans were Catholic.
"German communities often toned down their ethnic displays, especially in the Midwest," O'Brien said. "The number of German-language newspapers in cities like Milwaukee dropped precipitously, and some school districts actually stopped offering German as a language elective for high-school students."
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"In some ways, World War I left many Catholics in a vulnerable position," O'Brien said. "The two largest ethnic groups who opposed the American entry into the war were German and Irish Americans, both of whom drew the ire of President Woodrow Wilson. Pope Benedict XV's call for a negotiated peace also prompted some Americans to question whether the Pope secretly favored the Central Powers (although there is no evidence for this)."
The war's Catholic critics have had little prominence in history, but their lives too are being revisited. Benjamin Salmon, one of the few Catholic conscientious objectors to the war who even declined a non-combatant role, was a strict foe of all war to the point where he seemed to reject Catholic just war theory.
His writings against the war were censored by the U.S. Post Office. The New York Times claimed he was a suspected spy, and other newspapers ridiculed him as a coward and a slacker. He was convicted in a civilian court, then convicted in a military court despite never being inducted as a soldier.
Salmon was initially sentenced to death, with his sentenced later reduced. He spent a long period in solitary confinement and his 135-day hunger strike resulted in forced feeding.
Finally released after a pardon in 1920, he had to move to Chicago with his wife and son to avoid animosity in Denver. He continued to be a devout Catholic, with a son and a daughter entering the priesthood and religious life, respectively. Some of his devotees asked the Archdiocese of Denver to consider opening his canonization cause in 2015, the Denver Post reported.
Men and women who sought to support the war effort included the Knights of Columbus, whose members provided relief work staff and helped raise more than $14 million for recreation centers for soldiers stationed in Europe.