The Americans

First there's a difference between adjective and plural noun. With adjectives, like French or Chinese, you have to use 'the' to refer to the people: The French eat a lot of garlic, and the Chinese eat rice.

With adjectival forms like Norwegian(s) and American(s), you can use either: (The) Norwegians like skiing.

It's hard to see much difference in generalizations like these. Perhaps there's a bit of a sense that when you ass 'the' you're speaking about more of them, or the nation as a whole rather than just a typical group of them.

Of course without 'the' it can mean 'some', not necessarily a big number: Irish fought in the Second World War (e.g. with the RAF), but you can't say The Irish fought..., because the country wasn't involved.

me again. *cough* ass -> add in third para. Ta.