The painter Childe Hassam (1859-1935) was an influential proponent of impressionism in the United States. A decided Francophile, he was appalled by the German invasion of France in 1914 and responded with "the Flag series," the paintings that were to become his best-known work.
Even before the United States entered the war, Hassam was already painting the American flag alongside those of the Allies, portraying for instance a 1916 "Preparedness Parade" in New York City. The political message could not have been less subtle.
Lots of impressionist painters, including Monet, have incorporated flags in their work. It makes sense: flags are, after all, actually designed to capture and reflect light, and so likely to capture the imagination of artists setting out to capture the play of light.
Hassam's flag paintings are only unusual in that they combine masterful impressionist technique with fervent nationalist feeling.