Great Works: Mars, c 1638, by Diego Velázquez

Prado, Madrid

According to Lemprière's Classical Dictionary, that book so beloved by the English poet John Keats, the Romans adored Mars, the god of war. They regarded him as the patron of their city and the father of the first of their monarchs.

Augustus built a great temple in his homage after the battle of Philippi. "His altars," we read, "were stained with the blood of the horse, on account of his warlike spirit, and with the blood of the wolf, on account of his ferocity. Magpies and vultures were also offered up to him, on account of their greediness and voracity..."

How do we square all that praise and exaltation then with the pitiful, serio-comic image on this page painted by Diego Velázquez, court painter to Philip IV of Spain? Consider for a moment where this painting was hung – in the king's hunting lodge. Hunting was a leisure pursuit, a tame variant upon martial activities. Here Mars has been thoroughly dragged to Earth.

He is not the muscular young hero we see in the celebrated Ludovisi Mars, for example. He is older. He has passed his best. He is deflated in spirit. This is the weakened aftermath of all those godly triumphs. And, rather unusually, the god is on his own, contemplating his own pitifulness. He seems to look out at us from somewhere deep at the back of his eyes, as if slightly adrift or bewildered or ashamed.

How did he come to such an unpretty pass as this? Well, it is in fact quite a pretty pass. The way in which that abundant pink drape laps up into his groin is quite coyly fetching, though also slightly humbling and perhaps even inappropriate in its prettiness. His aloneness here seems to be emphasised by the featureless, blankly enveloping, greeny-brown surround at his back.

His body seems to sag forward acceptingly, as if in acknowledgment of a humiliating state of weakness. Is this a moment of post-coital tristesse? Perhaps. Although we know almost nothing about Velázquez, his life, his work, his attitude towards his official duties, we do know that there is another painting by him that shows the god Vulcan being informed of Mars's treachery in bedding Vulcan's wife Venus. That painting is in the Prado too. Perhaps the painting on this page is an exercise in remorse then.

And what of the facture of this great work? Call it brisk and assured if you like, a kind of brilliant, cursory realism painted in the aftermath of Velázquez's absorption of lessons from such Venetian masters as Titian. Details are rendered with such speed and assurance – look at that flash of white on the big toe of his left foot, for example. And, near that same foot, there lies Mars's magnificent armour – he was so admired for the brilliance of his martial appurtenances. Now it too looks profoundly sad, heaped up there so untidily at the bed's foot, about to be consigned to the lumber room of history.

About the artist: Diego Velázquez (1599-1660)

The enigmatic Diego Velázquez, described by Manet as "the painter's painter", was the greatest of all Spanish painters. Born in Seville in 1599, he had five siblings. He had a mastery of still life and portraiture. Court painter to the Habsburg king Philip IV, his intense scrutiny of the monarch as he aged was just as pitiless as Goya's renderings of the Spanish royal family of his day, yet Velázquez did it with more subtlety and less overt cruelty.