Rulers have always known that people believe what they see. A marble statue found in 1863 at the villa of the empress Livia, at Prima Porta near Rome, shows her husband Augustus, the first Roman emperor, as a young general addressing his troops. He was in fact middle aged or older when such portraits circulated, yet he never ages in his official images. By his right leg rides a small Cupid on a dolphin. The Cupid beside Augustus of Prima Porta advertised the family's claimed descent from the goddess Venus, Cupid's mother. The statue is a careful political argument made of stone.

Images that make rulers

The Assyrian kings of the ninth to seventh centuries BCE lined their palaces with reliefs of lion hunts and conquered cities, reminding visitors of what resistance cost. In 1701 Hyacinthe Rigaud painted Louis XIV of France in ermine-lined coronation robes, showing off his legs in silk stockings and red-heeled shoes, a costume that was itself a sign of rank.

Few images are more calculated than Jacques-Louis David's Napoleon Crossing the Alps, painted from 1801. Napoleon sits calmly on a rearing horse, pointing upward, his cloak whipped by wind. The names of Hannibal and Charlemagne, earlier conquerors who crossed the Alps, are carved into the rocks below. In reality Napoleon made the crossing in May 1800 on a mule, not on a rearing horse, in fair weather. He asked to be shown "calm on a fiery steed," and David obliged.

Images that accuse

Art can also turn against power. The Spanish painter Francisco Goya was a court painter, yet after French troops occupied Spain in 1808 he made some of the most unflinching images of violence ever produced. His painting The Third of May 1808, made in 1814, shows a French firing squad executing Spanish civilians. Goya painted the soldiers as a faceless row, while the victim at the center, in a white shirt, raises his arms as if on a cross. The killers are a machine; the victim is a person.

Goya also made a series of more than eighty etchings, now called The Disasters of War, showing atrocities by every side: mutilation, rape, famine. The Disasters of War were published only in 1863, decades after Goya's death, because they were too dangerous to release. Under one image he wrote simply, "I saw this." Pablo Picasso later took up Goya's role: Picasso painted Guernica in response to the bombing of a Basque town in 1937, and he refused to let it go to Spain until democracy was restored.

States that control art

Twentieth century dictatorships treated art as a weapon. In 1937 the Nazi government seized thousands of modern works from German museums. The Degenerate Art exhibition, held in Munich in 1937, displayed seized modern works in order to mock them, hung crookedly with insulting slogans and labels showing what museums had paid. More than two million people visited, far more than saw the official show of approved art nearby. Many artists in it, including Jewish artists, were driven into exile or killed.

In the Soviet Union, from the early 1930s, socialist realism was the official style, requiring art that was easy to understand, optimistic and in service of the Communist state. Painters showed heroic workers, abundant harvests and a kindly Stalin. Artists who did not conform could lose their careers, their freedom or their lives.

Democracies argued over art and power too. In 1933 the Mexican muralist Diego Rivera, commissioned to paint a mural in New York's Rockefeller Center, included a portrait of Lenin. When he refused to remove it, the mural was covered and in 1934 destroyed.

Worked example: Guernica

On 26 April 1937, during the Spanish Civil War, German and Italian aircraft supporting General Franco bombed the Basque town of Guernica for several hours. The town was largely destroyed and many civilians died; estimates have ranged from a few hundred to more than a thousand. Picasso painted Guernica in response, completing it within about five weeks for the Spanish Republic's pavilion at the 1937 Paris International Exposition.

Form: the canvas is huge, about 3.5 meters high and nearly 8 meters wide, and painted only in black, white and grey, like a newspaper photograph. Shapes are flattened and broken in the Cubist manner, and the composition is a jagged mass of bodies under a harsh light bulb shaped like an eye.

Content: on the left a woman screams to the sky holding a dead child, beneath a bull. In the center a horse, pierced by a spear, opens its mouth in agony. A dead soldier lies at the bottom with a broken sword, near a small flower. From a window a woman thrusts a lamp into the scene, as if to make the world look. No airplanes appear. The painting shows not the attack but what it does to living bodies.

Context: Picasso kept the painting out of Franco's Spain, and it spent decades in New York at the Museum of Modern Art. It arrived in Madrid in 1981, six years after Franco's death. It has since become a worldwide emblem against war. In this sense Guernica belongs with Goya: it insists that the people under the bombs are not abstractions.