美国意大利黑帮 - Expressing location without 在?
Chinese and English express this in different ways. Or better yet, English emphasizes the nuance that these mafiosi are of Italian descent but operate in America: Italian mafia in America.

The Chinese term 美国意大利黑帮 is just "American-Italian" mafia. Chinese has no proper word for demonym-adjectives, Italian will translate as 意大利人 if it is a demonym denoting a person ("Italy-person") and most likely 意大利的 if it is something from Italy (e.g. 意大利的经济 Italian economy) but omitting 的 is more natural in most cases, e.g.  意大利经济 Italian economy. It's a bit like headline style in English: UK minister resigns (where UK is certainly not an adjective) and not British minister resigns. 

Since Chinese heavily relies on context, 美国意大利黑帮 requires you to use your knowledge and understanding of the context, and put this together. English on the other hand prefers splitting this up and making clear what role Italian and American play here.