Ludovico Sforza, Leonardo's employer, was actually a regent ruling in the stead of the rightful Duke of Milan. When the young Duke came of age, Ludovico refused to hand over power, so the rightful Duke got the King of Naples to support him.

In an unfavourable position, Ludovico panicked and appealed for help from outside Italy, inviting the French in with promises of support for their King's claim on Naples. In the ensuing chaos (which included him turning against the large French army), his challenger (the real Duke) was poisoned and the King of Naples (the real duke's sponsor) died. It looked good briefly, until the French king died. His successor, Louis XII, turned out to be related to the recently-assassinated young Duke (which importantly meant he had a hereditary claim to Ludovico's position).

Ludovico had by now irritated almost everyone. He had set a precedent for French inclusion in Italian warfare, irritated most of his neighbours (including the Pope) and betrayed the French army he had promised to support.

When the French -- already a dominant military power -- came for Milan, they had the backing of both Venice and the Pope. Ludovico just fled, Milan barely attempted to defend itself.

Leonardo stayed in French-occupied Milan for a while, before fleeing to Venice to take on some work there. Ludovico would be captured later, during an attempt to re-take Milan from the French with mercenaries.

Edit: The point involving Leonardo is that even if he had been capable of coming up with something crazy that made Ludovico more likely to win a few battles, the political situation was just too poor for victory to seem plausible. Ludovico sensibly fled rather than struggle against poor odds, Leonardo was never called on to help him fight the French.