Picasso underwent surgery for a prostate condition in 1965, and the surgery is rumored to have left him largely impotent--if true, a serious handicap for a man to whom sexual adventure was such an important part of life. He seems to have increased his artistic output during this period, and some have suggested that this was his way of dealing with the condition.
Beginning in 1907, Picasso began to experiment with Cézanne's techniques alongside fellow artist Georges Braque. Cézanne was a constant touchstone for the two artists during this period of collaboration, which eventually resulted in the invention of Cubism. In 1943, Picasso would declare to photographer George Brassaï that Cézanne was "my one and only master."
Gertrude Stein became Picasso's principal patron, acquiring his drawings and paintings and exhibiting them in her informal Salon at her home in Paris. At one of her gatherings in 1905, he met Henri Matisse, who was to become a lifelong friend and rival.
During World War II, Picasso remained in Paris when the Germans occupied the city, but his artistic style didn't fit the Nazi ideal of art, so he did not exhibit during this time. He was often harassed by the Gestapo. During one search of his apartment, an officer saw a photograph of the painting Guernica. "Did you do that?" the German asked Picasso. "No," he replied, "You did".