 His private clients included Alfred H. Barr, Jr., Chester Dale, Douglas Dillon, and Marjorie and Duncan Phillips, who through purchases from Rosenberg created much of the modern collections within The Phillips Collection in Washington, D.C..In the late 1930s, Rosenberg, alert to signs of an approaching war, began quietly moving his collection out of continental Europe to the London branch and to storage in America (via the 1939 New York World's Fair), Australia and latterly South America.He then stopped adding to his collection in France, and advised his artists to make similar arrangements.