Picasso's African-influenced Period (1907–1909) begins with his painting Les Demoiselles d'Avignon. The three figures on the left were inspired by Iberian sculpture, but he repainted the faces of the two figures on the right after being powerfully impressed by African artefacts he saw in June 1907 in the ethnographic museum at Palais du Trocadéro.  When he displayed the painting to acquaintances in his studio later that year, the nearly universal reaction was shock and revulsion; Matisse angrily dismissed the work as a hoax. Picasso did not exhibit Les Demoiselles publicly until 1916.  (excerpt from Wikipedia)

Despite the dates given above, this is an art style that Picasso would return to years later and it influenced not only his future works, but those of many other artists and its usually the painting that people point to as the origins of Modern Art. 

Picasso once said that this painting should be called, "the death of Pichot". He said this because he painted it after his good friend Ramon Pichot died. He had been painting "like a respectable painter" for several years due to his marriage with Olga Khokhlova who was the daughters of a Russian General. This painting would signify his return to his 'modern art' style and the end of his marriage.

Arguably Picasso's most famous work is his depiction of the German bombing of Guernica during the Spanish Civil War – Guernica. This large canvas embodies for many the inhumanity, brutality and hopelessness of war.  (excerpt from Wikipedia)

This is another example of this style which was first exhibited in 1937. It is massive in size, 11.5 feet tall by 25.5 feet long.