Looking at artistic depictions of conflicts from the late 18th century such as the American Revolutionary wars and the French Revolutionary Wars and associated conflicts, most common line soldiers seem to be equipped with bicorne and tricorne (1,2,3,4) style hats with some exceptions, such as bearskins.
However, at the end of the Napoleonic wars in 1815, all major combatants seem to have to some extent equipped their line infantry instead with the tall, brimmed shako caps (Examples from France, Austria, Prussia, Russia, Great Britain, Italy and Spain), with the bicorne mostly to bee seen worn by commanders (such as famously by Napoleon himself), and the bearskin still seeing some use. What was the reasoning for seemingly such a significant surge in popularity of this headwear? Was it tied to Napoleons great sucesses in the earlier coalition wars and subsequent influence over Europe?
I've written about this previously here.
TLDR - By the 1760's it was obvious Tricornes were quite impractical and European armies experimented with various replacements up until the French Revolution, mostly leather helmets which didn't work out. Light troops popularised shakos during the 1790's, which were adopted generally after they were found to be practical, attractive and - more importantly - cheap.
