Georges Braque, Landscape of l'Estaque, 1907, oil on canvas, 37 x 46 cm. (Musée d'Art moderne, Troyes, France)
During the summer of 1908, Braque returned to Cézanne's old haunt for a second summer in a row. Previously he had painted this small port just south of Aix-en-Provence with the brilliant irrevent colors of a Fauve (Braque along with Matisse, Derain, and others defined this style from about 1904 to 1907).  But now, after Cézanne's death and after having met Picasso, Braque set out on a very different tack, the invention of Cubism.
Cubism is a terrible name. Except for a very brief moment, the style has nothing to do with cubes. Instead, it is an extension of the formal ideas developed by Cézanne and broader perceptual ideas that became increasingly important in the late 19th and early 20th centuries.  These were the ideas that inspired Matisse as early as 1904 and Picasso perhaps a year or two later. 
Pablo Picasso, Les Demoiselles d’Avignon, 1907, oil on canvas, 8 x 7 feet and 8 inches (Museum of Modern Art, New York)
We certainly saw such issues asserted in _Les Demoiselles d’Avignon._But Picasso’s great 1907 canvas is not yet Cubism.  It is more accurate to say that it is the foundation upon which Cubism is constructed. If we want to really see the origin of the style, we need to look beyond Picasso to his new friend Georges Braque.

A New Perspective

The young French Fauvist, Georges Braque that had been struck by both the posthumous Cézanne retrospective exhibition held in Paris in 1907 and his first sight of Picasso’s radical new canvas, Les Demoiselles d’Avignon. Like so many people that saw it, Braque is reported to have hated it—Matisse, for example, predicted that Picasso would be found hanged behind the work, so great was his mistake. Nevertheless, Braque stated that it haunted him through the winter of 1908. Like every good Parisian, Braque fled Paris in the summer and decided to return to the part of Provence in which Cézanne had lived and worked. Braque spent the summer of 1908 shedding the colors of Fauvism and exploring the structural issues that had consummed Cézanne and now Picasso. He wrote:
It [Cézanne's impact] was more than an influence, it was an invitation. Cézanne was the first to have broken away from erudite, mechanized perspective…[1]
Paul Cézanne, Mont Sainte-Victoire, 1902-04, oil on canvas, 28 3/4 x 36 3/16 inches (73 x 91.9 cm) (Philadelphia Museum of Art)
Like Cézanne, Braque sought to undermine the illusion of depth by forcing the viewer to recognize the canvas not as a window but as it truly is, a vertical curtain that hangs before us. In canvases such as Houses at L’Estaque( 1908), Braque simplifies the form of the houses (here are the so called cubes), but he nullifies the obvious recessionary overlapping with the trees that force forward even the most distant building.
Georges Braque, Houses at l'Estaque, 1908, oil on canvas, 73 x 60 cm. (Kunstmuseum Bern, Bern, Switzerland)

Brothers of Invention

When Braque returned to Paris in late August, he found Picasso an eager audience. Almost immediately, Picasso began to exploit Braque’s investigations.  But far from being the end
Pablo Picasso, The Reservoir, Horta de Ebro, 1909, oil on canvas, 24-1/8 x 20-1/8 in. (Museum of Modern Art, New York, fractional and promised gift)
of their working relationship, this exchange becomes the first in a series of collaborations that lasts six years and creates an intimate creative bound between these two artists that is unique in the history of art.
Between the years 1908 and the beginning of the First World War in 1914, Braque and Picasso work together so closely that even experts can have difficulty telling the work of one artist from the other. For months on end they would visit each others studio on an almost daily basis sharing ideas and challenging each other as they went. Still, a pattern did emerge and it tended to be to Picasso’s benefit. When a radical new idea was introduced, more than likely, it was Braque that recognized its value. But it was inevitably Picasso who realized its potential and was able to fully exploit it.

Tough Art

Georges Braque, The Portuguese, 1911, oil on canvas, 116.8 x 81 cm (Kunstmuseum Basel, Basel, Switzerland)
By 1910, Cubism had matured into a complex system that is seemingly so esoteric that it appears to have rejected all esthetic concerns. The average museum visitor, when confronted by a 1910 or 1911 canvas by Braque or Picasso, the period known as Analytic Cubism, often looks somewhat put upon even while they may acknowledge the importance of such work. I suspect that the difficulty, is, well..., the difficulty of the work. Cubism is an analysis of vision and of its representation and it is challenging. As a society we seem to believe that all art ought to be easily understandable or at least beautiful. That's the part I find confusing.
Essay by Dr. Beth Harris and Dr. Steven Zucker
[1] As quoted in William Rubin, Picasso and Braque: Pioneering Cubism, New York: The Museum of Modern Art, 1989, p.353.

Additional resources: