Answer: The First World War

Veteran’s Day, also known as Armistice Day, has been observed annually on November 11th for almost a century. Though World War I did not come to an official end until the signing of the Treaty of Versailles in the Spring of 1919, on November 11, 1918, Germany and the allies came to terms over a cease-fire agreement in France.  The first "Armistice Day" was proclaimed a year later by President Woodrow Wilson, which marked the first anniversary of the 1918 accord. Up until World War II, the U. S., France, and Great Britain all recognized Armistice Day.

In the U. S. in 1954, the name of the celebratory day was changed to "Veterans Day" to honor American veterans of all wars. In similar veins, both the Great Britain and Canada celebrate Remembrance Day around the same time as Veterans Day.

Among HSP's collection are a number that relate to World War I, such as the F. Furman Betts papers (#3655), the Charles H. Sykes papers (#3656), and the Stephen H. Noyes papers (#1472). More images like the one you see above can be fond in the Philadelphia War Photograph Committee collection (#V03).