Some rivalries run deep. As long ago as 1260, while the Italian city-states were going through one of their periodic bouts of fractious struggle, the republic of Siena defeated the Florentines at the Battle of Montaperti. To this day, Sienese football supporters bawl, "Montaperti! Montaperti!", when ACF Fiorentina performs badly against their beloved Bianconeri.
Traditional hatreds played out on the football field do not usually matter very much in art galleries. But this case is different. The historic rivalry between Siena and Florence did carry over into the field of art. Sienese and Florentine artists set out deliberately to create competitive styles for painting and sculpture that declared the separate identities of the two cities. And the fact that, in the end, the Florentines triumphed has affected our whole sense of what Renaissance art should look like.
In a few weeks' time, Italian Renaissance paintings, drawings and sculptures from all over Europe and America will begin to arrive at the National Gallery for the exhibition Renaissance Siena: Art for a City. It is rather unlikely, though, that the names of even the greatest Sienese artists of this time - the 15th and 16th centuries - will trip off the tongue. The unexpected talents of artists such as Francesco di Giorgio and Domenico Beccafumi have long been eclipsed by their more famous Florentine neighbours. This will be the chance to show that the aesthetics of Siena and Florence may sometimes have been poles apart, but that Siena's art is no less lovely. It is simply beautiful in another way. And, if it has been largely unappreciated, it is merely that we have fallen victim to some carefully calculated Renaissance propaganda - emerging, one need hardly add, from Medici Florence.
In 1555, Florence took its final revenge for Montaperti, conquering Siena to form the Grand Duchy of Tuscany. At exactly this time, Giorgio Vasari, court artist and lickspittle at the court of Grand Duke Cosimo de' Medici, published two editions of his The Lives of the Most Excellent Painters, Sculptors and Architects. For Vasari, therefore, Tuscan art had to be Florentine. His history of art marched - rather relentlessly - from Giotto to Michelangelo, and many of the most imaginative artists of Renaissance Siena were quite simply written out.
Vasari made us believe that the Renaissance happened in Florence, and it is from him that we derive the idea that the history of art should be progressive, getting better and better as it becomes ever truer to nature. According to this view, people in paintings should look like people in life - though, of course, only the most aesthetically perfect. Biblical and mythological narratives should be played out on mathematically constructed, perspectively accurate stages - in real spaces.
Though this Florence-centric view is still taken as gospel by many, we are now beginning to realise that this is only one part of the story. There are other Renaissances waiting to be discovered - and the Sienese Renaissance is one of the most fascinating. There is no question that the art of Francesco di Giorgio and Domenico Beccafumi, let alone of their still more neglected peers, the delicate Neroccio de' Landi or the highly expressive Matteo di Giovanni, fits uncomfortably within the Vasarian paradigm. These were not artists who sought to make pictures of the "real" world. Instead, they often chose a visionary style, celebrating elegant, sinuous line and the exquisite use of gold. Their paintings provide glimpses of heaven.
Their art also expresses their pride in belonging to Siena. Beccafumi himself proclaimed that he could not work well unless breathing the air of Siena. Consequently, the National Gallery's exhibition will be as much about Siena's civic pride and history as about her individual artists.
Even today, Sienese pride is defined to an astonishing extent by its glorious Middle Ages and Renaissance - including, of course, its artistic heritage. Indeed, some of the most striking exhibits travelling to London will come from the city itself, many leaving for the first time. Together, they give an idea of Siena's unique - even defiant - continuity with its past.
Take, for example, the coloured wooden statue of Saint Catherine of Siena by Neroccio de' Landi. This was carved in 1474, soon after the 14th-century Dominican nun Caterina di Benincasa was canonised by a Sienese pope and adopted as one of the patron saints of her native city. She stands where she has always stood - on the altar of the church built on the site of her father's cloth-dyeing workshop, the oratory of the Contrada dell'Oca (the District of the Goose). This church is not usually open to tourists (except by appointment), and the statue remains central to the identity of this particular area of Siena, now and for six centuries acting as its spiritual heart. The heart is still beating and the loan to London of Neroccio's Saint Catherine was agreed only after the vote of a special district assembly, and members of the Contrada will be making regular inspections while she is visiting the National Gallery. Though so visually arresting, this sculpture is much more than just a work of art.
Two pictures by Matteo di Giovanni are coming to the National Gallery from a little, very pretty town south-east of Siena, Asciano, once under Sienese control. Like the Saint Catherine, these paintings work on several levels. They are wonderful examples of Sienese gold-ground painting. But once they have arrived in London, they will become something more. Here they will be rejoined to the enormous altarpiece to which they were once attached - to Matteo's picture of the Assumption of the Virgin Mary, bought by the National Gallery in the 19th century.
On the eve of Montaperti, the Sienese dedicated their city to the Virgin. Ever since, the Feast of the Assumption, on 15 August, has become the most important day in Siena's civic calendar. It was believed - is still believed - that the Virgin offers her special protection to Siena; the dedication ceremony has been repeated several times - most recently during the Second World War, as bombs were dropping nearby. Seeing this great altarpiece reunited will clearly convey the powerful presence of the Virgin of the Assumption in the daily lives of the city, its citizens and its territories. Again, this painting can be viewed as both an act of faith and an artistic masterpiece.
These are but two examples among many. There will be more than a hundred paintings, drawings and sculptures of this kind - pictures of ancient heroines based on the celebrated Sienese beauties of the Quattrocento, sculptures proclaiming the legendary Roman foundation of the city, paintings reunited that once adorned the palace bedchambers of the patriciate, and images that proclaimed Siena's own brand of piety.
During the years covered by the show, from about 1460 to 1530, Sienese style might have been somewhat modernised, but its essential ingredients remained true to its original, exquisite gold-ground dialect. Notwithstanding Vasari, Renaissance Siena: Art for a City will be a chance to rewrite art history a little.
Luke Syson is the curator of Renaissance Siena: Art for a City, which runs at the National Gallery, London WC2 (020-7747 2885) from 24 October to 13 January