"World War I and American Art," the ambitious and almost heartbreaking exhibition at the Pennsylvania Academy of the Fine Arts through April 9, begins and ends with flag-waving.

Childe Hassam's "Italian Day."

In the first gallery, five big paintings by Childe Hassam, all from 1917 and 1918, depict wartime New York as a spectacle of patriotism. The faces of the people in the crowds on Fifth Avenue don't have any individuality. The focus is on the dynamism of the hundreds of billowing flags that line the streets and signal national unity, power, and optimism.

The show ends with a similar scene, by George Luks, showing the celebration of the Armistice that ended the war on Nov. 11, 1918. It's an ecstatic picture, though again we don't really see the people as individuals; their emotions are in the flags of the victors, which seem to be floating high overhead.

In between these scenes of celebration are many different kinds of images. There are propaganda posters that show the enemy as subhuman monsters and Americans as sexy, energetic, and invincible. There are celebrations of the family and of motherhood, presumably values we were fighting to protect. We see artists and propagandists hoping that black soldiers' participation in the war will lead to greater dignity and respect for their people.

There are expressions of dissent, like the cartoon in which a skeleton representing the newly instituted military draft system measures a young man and declares him physically fit - for a coffin.

In the same gallery as the Hassams are three paintings by Marsden Hartley that are also about symbols and images. But these are what remains of the artist's memory of a love affair with a German officer who died early in the war, long before America was involved. They have no face in them; the beloved is dead, and these medals and memories of monuments are what's left.

We see the artist Charles Burchfield, stricken with anxiety that he will be drafted, doing drawings and watercolors that try to explain his dark feelings, and which turn rural Ohio into a vision of no-man's-land.

There are many aerial photographs, some by the distinguished photographer Edward Steichen, of ruins, trenches, and countryside. And there are a couple of paintings by John Marin that appear to be directly influenced by this new way of seeing the landscape. Toward the end, we see the wounded, as well as documentation of the work of sculptor Anne Coleman Ladd, who created masks so the permanently disfigured could show a face to the world.

And at the center is the battlefield, brutal and chaotic, where the pageantry of the military parades back home is long forgotten and men must suffer and die alone.

The exhibition was organized by PAFA under the direction of its former senior curator Robert Cozzolino, now at the Minneapolis Institute of Art; independent scholar Anne Knutson; and David Lubin, professor of art at Wake Forest University. It is the first ever done on the subject.

"World War I was the most complex event in human history," declares a text panel near the beginning of the show. Why, then, should we have waited for the centennial of the event for a show like this?

One explanation is that although the war lasted four seemingly endless years for most of its combatants, direct American involvement lasted only about a year and a half. It was seen as, at most, an interlude, rather than the epochal event it was in Europe. Moreover, with a few exceptions, American art of this period has not been considered all that important.

The curators go to some lengths arguing that works by some well-known artists who are not associated with the war - such as Marin and Georgia O'Keeffe (whose brother died from the after-effects of poison gas) - do reflect wartime imagery.

John Singer Sargent is best known for his society portraits, but he did the exhibit's most monumental work, Gassed (1919), which shows soldiers temporarily blinded by mustard gas, blindfolded and leading one another around - the blind leading the blind. Others lie struggling to recover in the foreground.

Gassed, by John Singer Sargent.

The 20-foot-wide canvas is so large and carefully worked you almost feel you can walk into it. Nevertheless, the more I look, the more distant I feel. Is it likely that every one of the soldiers could have been so good-looking? It is a harrowing scene, but Sargent couldn't help making it a bit glamorous.

John Singer Sargent's "Crashed Aeroplane."

Crashed Aeroplane (1918), one of several watercolors Sargent made during a wartime tour, shows a farmer laboring in the foreground with the hulk of a crashed biplane behind him. It echoes Pieter Brueghel's famous painting of the fall of Icarus, acquired by the Brussels Fine Arts Museum in 1912. Sargent visited the war to make art.

These technically polished works contrast strongly with the works of those who were sent to war to fight and who then made art. Horace Pippin had none of Sargent's finesse, and the war left him injured both physically and psychologically. But a decade later, he was able to distill his experiences into some unforgettable works.

I was most moved, however, by an artist I had never heard of: Claggett Wilson (1887-1952), who served in the Marine Corps and fought in the bloody battle at Belleau Wood. The watercolors he made the year after the war record specific moments in battle, showing soldiers on both sides. Some have highly specific descriptions and dates of the battle, and the works vary a good deal in style. What they share is immediacy and intense emotion.

Claggett Wilson's "Flower of Death - the Bursting of a Heavy Shell-Not as It Looks, but as It Feels and Sounds and Smells."

Flower of Death - the Bursting of a Heavy Shell-Not as It Looks, but as It Feels and Sounds and Smells (c. 1919) seems to be an abstraction until you see the barbed wire and the face in the corner. Front Line Stuff shows a couple of almost cartoonish soldiers in the foreground, but they are headed into a space that is mysterious, terrifying, and sublime. Wilson's war is a series of horrific, potentially transcendent surprises.

This was to be the war to end wars. Instead, it turned out to be a prototype of industrialized total war. If you think it is a distant and irrelevant event, this extraordinary show will convince you otherwise.

World War I and American Art: Through April 9 at Pennsylvania Academy of the Fine Arts, 128 N. Broad St.

Hours: Tuesday to Friday, 10 a.m. to 5 p.m.; Wednesday, until 9 p.m.; Saturdays and Sundays, 11 a.m. to 5 p.m. Admission: Adults, $15; seniors (60 & over) and students, $12; youths, (13-18) $8; children (12 & under), free. During the exhibit, free on Sundays. Information: 215-972-7600 or www.pafa.org/museum