The first American invasion of Normandy was an exciting but peaceful affair that took place nearly a century before the landings of World War II.

Led by James Abbott McNeill Whistler, who was capturing the coasts of Brittany and Normandy on canvas while the Civil War raged at home, American artists roamed Western France in search of light, air, painterly subjects and cheap living.

For half a century the Yanks reveled in the rugged rural landscape, the virtually medieval local costumes and customs and, especially, that cheap living. Villages such as Pont-Aven, Concarneau and Giverny became American art colonies. The brash and pestiferously worshipful American boys all but drove Claude Monet from his home and famous gardens at Giverny, and while he was successfully thwarting the Yankee suitor of his stepdaughter Blanche, another American carried off stepdaughter Suzanne.

And so it went until the eve of the First World War. The flavor of it all is laid out for us in an entrancing exhibition that has been touring the country since last fall and now is making its last stop at the Smithsonian's National Museum of American Art.

Assembled by David Sellin and James K. Ballinger of the Phoenix Art Museum, the exhibition's installation here has been forcefully designed by Georgine Reed to demonstrate the progress of the American infiltration of the countryside, both geographically and stylistically. The order is generally chronological, but where the artists don't fit neatly into rank, Reed has opened cross-galleries, so that paintings from different artists working in different times and places are allowed to resonate with each other. The 113 works by 71 artists are necessarily uneven because the exhibition strives to be representative.

It's a show of light, the light that drew the painters. We see Paris-schooled youngsters, emerging from skylit ateliers, first stagger and then exult in the plein air. In a passage from the show's excellent catalogue, Dennis Miller Bunker (1860-1891), describes the impact in a letter to a friend:

In sunlight it is never the same, the shadows which seem to be nearly in the same places when you are not painting them, really move with dreadful swiftness when you are. . . .

Many of the artists dodged from shadow to shadow or ducked back into salons, but eventually most grew bold enough to stand uncovered in the noonday sun, even taking nude models outdoors. From room to room in this show, the colors grow steadily brighter and the styles less studied, until one senses that an explosion is coming. It did come, and when the screams of outrage had died away it was termed Modernism. But that's a different story. AMERICANS IN BRITTANY AND NORMANDY, 1860- 1910 -- Through August 14 at the National Museum of American Art (Gallery Place Station on Metro's Red and Yellow lines). Open 10 to 5:30.