Galileo Galilei, one of the world's most famous scientists, mathematicians, and inventors, kept his favor with the Venetian court by inventing and peddling items for the Venetian military, especially his famous telescope.
See, there are two bits of information about Galileo's invention of the telescope in 1609 that some history books leave out. To start, he wasn't the first inventor of the telescope. A Dutch spectacle maker invented it before him, and Galileo may have even seen that telescope before he invented his.
Second, one of the first things that Galileo did with his telescope was to send it to the Doge of Venice, one of the republic's senior leaders, with the recommendation that it be used by the country's army and navy as an instrument of war.
Galileo Galilei created an 8-9x magnification telescope that he showed off the Venetian leaders. (Photo: Fresco by Giuseppe Bertini, Public Domain)
While Galileo might or might not have invented the first telescope, he almost certainly invented the most powerful one of his day. It was capable of an approximately 8-9x magnification at a time when everyone else reached only 4x.
That meant that Venetian admirals using a Galileo spyglass could have reconnoitered enemy fleets and positions from 8 miles away, where they would be pinpricks to someone using a 4x telescope and invisible to anyone who didn't have a spyglass.
Galileo outlined this potential advantage in his letter to the doge, but the doge didn't immediately buy it for Venetian forces. Still, Galileo was rewarded for his work. His salary as a professor of mathematics at the University of Padua was doubled and he was granted the position of "professor for life."
The inventor, of course, went on to find other uses for a good telescope. Galileo invented a 20x telescope that allowed him to identify the larger moons of Jupiter, the phases of Venus, and other phenomena in the night sky.
Galileo's "gunner's compass." (Photo: Sage Ross CC BY-SA 3.0)
The telescope wasn't the only thing that Galileo ever created for the military. He also created an improved "gunner's compass" that allowed artillerymen at the time to quickly calculate elevation, making them more lethal in siege warfare.