Answer:
The central idea of the passage is that pizza may be popular, but is still relatively new to the country.
We can infer that the American soldiers in Italy in WWII were influenced by the Italian cuisine present, and served as a driving factor for pizza being brought back into the United States and it's huge gain of popularity, being symbols of strength.
A quote supporting this is "Many American soldiers that had been in Italy during the war enjoyed eating pizza on a regular basis there and when they returned to the United States they wanted to continue to have access to it. Pizza then enjoyed a boost in popularity..."