Rubens, the artist who helped to shape the future of Europe Charles I was a keen art-lover, an avid collector and patron who attracted the likes of Orazio Gentileschi, his daughter Artemisia and Rubens's own pupil van Dyck to settle here. Rubens was a particularly shrewd choice of envoy, then, on Spain's part. The gift to Charles of Minerva Protects Pax from Mars, his purpose-painted allegory of war and peace, would hardly have harmed Spanish interests. Rubens's stay in Britain lasted nine months, during which time he worked on another, even more toadying, canvas: A Landscape with St George and the Dragon. Set by a corpse-ridden bank of the Thames, it's a fantastical vision of our patron saint, having slain the fire-breathing dragon, rescuing a fair princess. George is unquestionably modelled on Charles I, while the princess represents the peoples of England, and indeed Europe, of whom he's the saviour. Rarely have art and high-level diplomacy been so indelibly intertwined. There's a fascinating sense to Rubens's London works that the future of the continent is being played out within them. The charge often levelled at Rubens is that, for all his florid Baroque brilliance, his art lacks the suffering of, say, a Caravaggio or van Gogh to be truly great - yet how could anyone accuse these works of being merely ornamental? This was painting as a diplomatic weapon: conceived, created and wielded by one extraordinary man. While in London, Rubens also painted the portraits of many society luminaries - such as the haughty Earl of Arundel and the wise-bearded, royal physician Théodore Turquet de Mayerne. By March 1630, Rubens had won over king and court, and returned to Belgium with both a knighthood and peace accord. Yet his brush with England was far from over. Following his coronation in 1603, James I had launched a host of building campaigns. For one, he wanted the rickety old Banqueting House completely rebuilt, and to that end Inigo Jones, in 1621, began work on the Palladian edifice still standing today (the rest of Whitehall Palace was lost to fire in 1698). Rubens was approached by James to paint its ceiling, the ever-confident painter declaring: "My talent is such that no undertaking, however vast in size or diverse in subject, has ever surpassed my courage." No firm plans were fixed, though, and Rubens was soon at work in Paris - for four years - flattering another royal, the dowager queen of France, Marie de" Médici, with a monumental 24-canvas series at the Palais de Luxembourg. It wasn't till Rubens's diplomatic trip to London in 1629 that the Banqueting House project took off. James was now dead, and Charles I felt the finest commemoration would be to glorify his father (and, at the start of his 11-year break from Parliament, royal absolutism itself) on the Banqueting House ceiling. Once back in Antwerp, promised the regal sum of 3,000 florins, Rubens beavered away at the nine paintings, which would appear in a three-by-three grid. Bearing the distinct influence of Veronese, Tintoretto and Michelangelo, the ceiling remains in situ to this day. The focus of the Tate's exhibition, though, is Rubens's first sketch for it: discovered in 1956 and bought by the Tate, to considerable fanfare, for £5.7 million in 2008. Rubens was always a fine draughtsman, and this oil sketch combines, on one panel, plans for seven of the ceiling's nine eventual compartments. It may be painted in muted browns and whites, and lack the orgiastic pink flesh we associate with his canvases, but the sense of movement and energy is trademark Rubens. The central and largest scene is the oval-shaped Apotheosis of James I (also the centrepiece of the final ceiling). An eagle-borne James is assisted heavenwards by the figure of Justice, where the goddess Minerva awaits him with a crown. To his left and right (and each given their own separate section on the final ceiling) are allegorical personifications of the king's four main virtues (Temperance, Bounty, Strength and Wisdom) all triumphing over their bitter adversaries (Wantonness, Avarice, Discord and Ignorance). They encroach upon the ascending James's oval like magnets, swept towards heaven in a whirling, thrusting spiral. The sketch, in its own right, is a masterpiece of Baroque dynamism. It is at once a historical and artistic treasure, marking not just the apotheosis of James, but also that of Charles as a patron and Rubens as an artist. At £5.7 million, I'd say the nation bagged itself a bargain. To May 6; www.tate.org.uk This review also appears in SEVEN Magazine free with The Sunday Telegraph, on January 8, 2012.