American mixed drinks all filtered down from Punch with many of the drinks being English derived with some French influence before the American Civil War. The German style of drinking such as Cups also played a role around this time, and there was certainly a Dutch influence with gin. However, before the 1860s, there was little Italian influence for two reasons: one, there was not a lot of industry and exports from the region, and two, Italy was not even unified into a country back then. However, Italian drinking culture goes back 2500 years. During the late Middle Ages, Italy discovered distillation and started using it for medicinal cure-all potions that morphed into aperitifs and digestifs.
In the American style of drink making, the bartenders mix the libation for you, but in Italy, the bars have pre-bottled drinks. Italy learned to adapt with their exports. In a 1905 report on the American market, they observe that bartenders are mixing Italian cordials and vermouths with bitters and spirits, so the country needed to send something mixable and not the Italian traditional style. This Italian way included the 1869 Vermouth Cocktail that was simply vermouth, ice, and a lemon peel since the vermouth had the sugar and bitters already pre-bottled in them; other Vermouth Cocktail recipes had the addition of bitters. Martini & Rossi built the market and had about two-thirds of it, but the remaining third was still significant for the others.
Italian-influenced drinks included the Fernet Cocktail from 1906 Louis' Mixed Drinks where with a splash of a digestif in sweet vermouth, the drink becomes an aperitivo:
Fernet CocktailThis recipe was also very similar to William Schmidt's 1891 Appetizer a l'Italienne which utilized absinthe and gum syrup instead of curaçao.
• 2 liqueur glasses Italian Vermouth (2 oz Sweet Vermouth)
• 1 liqueur glass Fernet Branca (1 oz)
• 2 dash Curaçao (1/4 oz)
Stir with ice and strain into a cocktail glass.
At some point, soda water began to enter into the picture to lighten things up and make it all less sticky, and thus, the Americano was born. For example, there was a 1913 German drink book with an Americano of vermouth, Fernet, and seltzer. Indeed, the first Americanos were not Campari, but they eventually came to own the drink name through persistence, not by origin.
Of course, the story of how the Americano made with gin instead of soda water spawned the Negroni came up. There was definitely a Count Negroni who came up with the drink in 1920 but it was never published until around 1950. However, there were plenty of other drinks with different names but similar formulas. The 1926 Campari Cardinal which was lighter on the Campari had the same basic idea as did the 1927 Julio Cocktail. Similar drinks like the Boulevardier in Harry McElhone's 1923 Harry's ABC of Mixing Cocktails and 1927 Barflies and Cocktails and the Old Pal in the latter of Harry's books have a Negroni-structure but with a whiskey base. When any of those other drinks were first invented (versus first published) is a mystery, so it is uncertain which came first.
The war during the late 1930s to mid 1940s put a crimp on Italian ingredients entering the United States. In 1943, Italian ingredients came to Americans by way of the Allied troops entering Italy, and the soldiers were developing a taste of the Italian alcohols (the only thing available) as well as a hatred of the stuff (it reminded them of the war).
As for the question of garnish, the French preferred lemon while the Italian preferred orange, and there are drinks that call for different garnishes in different books.