Some large retrospectives feel like mountainous territory, peak after peak in an artistic life filled with triumphs. The National Gallery of Art’s large exhibition devoted to the work of George Bellows, the first major Bellows show since a 1992 painting retrospective in Los Angeles, feels more like pieces of a puzzle, a fascinating, imperfect puzzle. And when you have finished exploring the diversity of an American artist who died tragically young at the age of 42 in 1925, you may not feel as if the puzzle is quite complete.

But if you emerge willing to think pluralistically about modernism — that there was no monolithic Modernism with a capital M, but rather a host of different modernisms often in conflict with one another — then the work of Bellows will make more sense, and the curators will have accomplished their primary goal. As with so many artists, poets and composers who lived during periods that seemed (at the time) wildly eclectic and are considered (decades or centuries later) historically transitional, Bellows is unclassifiable. If you ignore large tracts of his work, he can, at best, be pigeonholed. This exhibition demands that Bellows’s oeuvre be considered as a whole, including a room of deeply disturbing paintings he made in response to World War I, political imagery that veers toward social caricature, and portraits in which the artist seems to be channeling the ethos of another era. In other words, a career of many modernisms.

When Bellows died of infection from a ruptured appendix in 1925, he was the most celebrated American artist of his age. A quarter-century later, when abstract expressionism was heralded as the first great, truly American artistic movement, Bellows was remembered primarily for a handful of paintings he made early in his career. Among these were the boxing works, “Stag at Sharkey’s” and “Both Members of This Club” (both from 1909) and “Club Night” (from 1907), in which boxers were depicted in the dynamic throes of combat, looming over the vicious and stupid faces of spectators, with arms and legs interlocked, like snapshots of statues by Rodin brought suddenly to life. The apparent speed in what is depicted and in the way the paint is put on the canvas, and the thrilling economy of gesture (musculature is limned with a few precise streaks of shadow and color), made these works seem the most “modern” of Bellows’s career — if one defined modern from the perspective of abstract expression and subsequent taste.

But Bellows painted many other things, and his early career is filled with stunning work that feels closely connected to the boxing pictures. His images of the excavation and construction of Pennsylvania Station have a similar minimalist expressionism, studies in mood and light that are more than the sum of their parts. They do for urban geology what Chaim Soutine did for meat. Seascapes from 1913, painted while Bellows was a resident in Maine, have a tempestuous energy every bit as vital as the boxing works. And early paintings of New York, including a 1911 canvas stuffed with people, horses, buildings and conveyances, suggest an artist feverishly exploring aesthetic breaking points: How much is too much? How little is not enough? Where is the line between the suggestive and the meaningless daub of paint?

And: How much weirdness can a painting convey without seeming merely garish, or a joke on the viewer? As Bellows matured, his output became stylistically more diverse, and at times the only common thread seems to be a distinctive quirkiness, a restless need to add in something weird and unpredictable. It is almost intangible, more a matter of the personality behind the art than what any individual piece says. Only a large retrospective gives enough data points to reveal this inchoate sensibility, and, for that, one is grateful for a chance to see Bellows from brilliant beginning to premature end.

Two dates inevitably configure our sense of Bellows’s career. One is his death in 1925, before the Great Depression, before the emergence of the American regionalist painters in the 1930s (Grant Wood and Thomas Hart Benton key among them), before World War II and before the mid-century revolution in American art. Both Bellows and Edward Hopper were born in 1882, but Hopper lived into the 1960s. What if Bellows had been given at least his three score and ten?

The other date in Bellows’s life is 1913, when the legendary Armory Show dazzled New York with an encyclopedic list of contemporary French and European art. Critical views of Bellows often center on this date as a turning point in the artist’s career, after which he turned to more traditional methods, with a vitiated output.

Curator Charles Brock will have none of that kind of thinking. Rather, he sees Bellows as consistently protean, not turning away from the Modernism of his early career but moving into a range of modernisms that make him a quintessentially American figure: energetic and experimental, absorbent like a sponge, unwilling to be classified and delimited. In a catalogue essay, Brock compares Bellows to Willem de Kooning, which at first seems preposterous, but try to get the idea out of your head while visiting the exhibition.

You don’t have to believe that it was all downhill after 1913 to acknowledge that something big was changing in Bellows’s work on or around that date. In the 1920s, lithography played a larger role in his output, and there is much to admire. But while the DNA traces of the delicious Bellows weirdness are every bit as apparent, there is a social condensation in the work that often feels ungenerous. Characters become stereotypes, and dramatic moments sometimes feel a bit too plotted and convenient.

The wartime paintings, large-scale works that depict German atrocities, strive for the visceral power of Francisco de Goya’s “Disasters of War.” But the more directly they confront the viewer — as in the 1918 “The Germans Arrive,” which shows a shirtless boy with his hands cut off by thuggish German soldiers — the more they seem to be about sadism rather than the barbarity of war. At the same time, something odd and terrifying is at work. The 1918 “Barricade,” in which naked Belgians are being used as human shields, is formally beautiful and psychologically disturbing, and every bit as powerful as the much later “Massacre in Korea” (1951) by Pablo Picasso, which it eerily prefigures.

No less discomfiting to any effort at pinning down Bellows are his portraits and figure paintings, many of them made in his Woodstock, N.Y., studio in the 1920s. If earlier rooms in the exhibition leave one wondering if Bellows simply couldn’t paint faces, here the artist paints them very well, capturing the claustrophobia of his sitters’ encounter with the artist, in a room with the shutters closed to daylight, and an old Victorian horse-hair sofa as a common leitmotif.

Bellows was a serious student of various theories of art and color, and anyone who wants to embrace the all and everything of Bellows’s career will have to struggle with some of the works apparently made under the influence of these ideas. What are the streaks of purple and green doing on the tender face of the girl depicted in the 1919 “Margarite”? Is this theory gone awry? More of Bellows pushing at aesthetic limits? Some lost species, a failure to evolve, in the generative flux of modernism?

None of these doubts detract from the pure pleasure of looking at Bellows’s art. If most visitors come away dazzled by the early works, and with a mixture of amusement and admiration for all the rest, the exhibition won’t necessarily have unsettled the long-standing view of Bellows. But that common wisdom about Bellows will at least be buttressed with a deeper knowledge of his work. And perhaps, over time, exhibitions such as this one will finally dismantle the still seductive view of monolithic Modernism.

George Bellows

is on view Sunday through Oct. 8 in the West Wing of the National Gallery of Art, Fourth Street and Constitution Avenue NW. .