From 1943 to 1946, Keller served as an officer of the U.S. 5th Army in the Monuments, Fine Arts, and Archives program. Captain Keller was primarily responsible for the identification and transportation of artworks in Tuscany, Italy, the center of the Italian Renaissance and a major site of Nazi looting. He worked in Pisa and Florence, where Nazi troops had bombed churches, destroyed or dislocated public art, and seized paintings from museums including the Uffizi. As they were discovered during the invasion of Italy, Keller repatriated lost works, including Giambologna's statue of Cosimo I de Medici, a fixture of Florence's Piazza della Signoria. He also documented damage to buildings and murals across Tuscany, taking thousands of photographs with Charles Bernholz.