Pizza is a traditional Italian dish
What are traditional toppings placed on the pizza?
There are many types.   It can also be topped with additional vegetables, meats, and condiments,
When was pizza first introduced to the United States?
After World War II, when Allied troops stationed in Italy came to enjoy pizza along with other Italian foods.
Did they bring any other Italian dishes with them?
A number of dishes.    It influenced several cuisines around the United States.
What was the first major pizza chain that opened after WWll
No first is known, but among the earliest was Little Caesars, founded in 1959 and based in Detroit, Michigan.
And they are still going strong. I had no idea they have been aroudn since 1959