A trio of people claim they invented the ice cream soda. But the good money (Wikipedia) says it was Philadelphia native Robert Green who in 1874 came up with the idea in order to steal customers away from a nearby vendor who had a bigger, fancier soda fountain.
Green mixed a scoop of vanilla with soda water and offered customers a choice of 16 different flavored syrups. They loved ‘em and over-night every self-respecting soda fountain had them on their menus.
But here is the weird thing. Soda was considered a miracle cure by many and so, was subject to control just like alcohol. In some places, dour conservatives managed to ban selling sodas entirely or at least on holy days. Not to be denied profit, the creative soda fountains owners came up with the soda-less ice cream sundae.
I love this story. It is triumph of Suh-weet over Sour.