Was the great Florentine artist and scientist who painted the Last Supper, designed the legendary Sforza horse and did the first great studies of human anatomy also a sinister military genius whose inventions caused horrible deaths in the wars of sixteenth century Europe?

This was a question I came up against in researching my book The Lost Battles: Leonardo, Michelangelo and the Artistic Duel that Defined the Renaissance.

The book tells how, from 1504 to 1506, these two titans of art were challenged to paint vast battle scenes in the Great Council Hall in the heart of Florence. Michelangelo, who was only 29 when their standoff started, had never fought in a war and, although verbally violent, seems to have been of a physically gentle and far from militaristic disposition.

Leonardo da Vinci on the other hand was fascinated by violent men - by the soldiers and warrior princes who dominated Renaissance Italian politics.

By the time he took on his commission to paint The Battle of Anghiari in the Florentine civic hall he was in his fifties and had worked nearly twenty years for the Milanese usurper Ludovico Sforza, "Il Moro", then for the warrior prince Cesare Borgia.

In a job application which survives from about 1480, Leonardo introduces himself to Sforza as a military inventor for whom art is a sideline.

He boasts of his superiority to other "masters of war" and offers to share with the ruler of Milan "my secrets", which include an armoured car, techniques for undermining fortresses, and other "means both of offence and defence."

Leonardo filled much of Manuscript B and other notebooks with such designs during his time in Milan, then after Sforza's fall proposed a daring submarine stratagem to the Venetian Republic before working with Borgia.

But do all the drawings of tanks, giant crossbows, fireships and even a steam-powered cannon in his notes bear any relation to reality?

Considering how many such drawings survive, they get shorter shrift than his more benign inventions - television programmes have reconstructed his parachute - but I have yet to see a modern reconstruction of his design for an incendiary dart with a "warhead" packed with combustible matter designed to blaze on impact.

It is even sometimes suggested that his war machines were satires or surreal entertainments. Perhaps this is because Leonardo da Vinci has become a hero of modern science: his curiosity was indeed heroic but perhaps he also anticipated the darker side of scientific knowledge, the lust for power over nature.

Or perhaps, then as now, that was the easiest way to get funding for research.

Stone and fire

In pondering the relationship between Leonardo's inventions and the reality of war in what was the first gunpowder age, I pursued a tradition of Italian military engineering that took up his example and was influential across 16th century Europe.

In 1588 - year of the Spanish Armada - a compilation of these Italian works was translated in London that - remarkably - mentions Leonardo's close associate Francesco di Giorgio as a pioneer of military science.

Leonardo worked closely with this Sienese polymath: it's tantalisingly close to seeing Leonardo himself cited by the English warriors who used fireships against the Armada.

And even without such a suggestive reference, the "fireworks" or incendiary weapons described in the book are very similar to some of Leonardo's. The many Italian works it draws on are full of engravings of such devices that simplify his brilliant designs.

Henry VIII's battleship the Mary Rose carried handguns with shields, made in Italy, that also resemble Leonardo drawings; a multi-barrelled gun in the Royal Armouries again uncannily resembled a Leonardo design. But, this is not surprising.

My point is not that Leonardo was some solitary guru to whom all these horrible Renaissance innovations can be traced back. Rather it is that he was a working engineer whose ideas are part of the mainstream of warfare in an age of disturbing change.

When the French invaded Italy in 1494 they brought cannon and their victory proved once and for all that gunpowder was the arbiter of battle: it hurt Leonardo directly as the bronze allotted to cast his giant statue of a horse in Milan was instead used to cast guns.

Leonardo and all his contemporaries were trying to make sense of the new artillery age and this was why they explored so many horrible incendiary devices.

Above all, Leonardo and - later in his life - Michelangelo designed fortifications whose capacity to resist artillery fire by being low, squat, and radically angled helped to transform defensive architecture throughout Europe.

Look at Renaissance forts and they resemble not castles, but modern concrete bunkers: here is the modernity of the Renaissance written in stone and fire.