Use of articles with nationality words (2)

Hi,

I think the use of the definite article with some nationality words is not clear cut. In some grammar books we can see the definite article always used before nationalities. Other grammar ones say that we do not normally use the definite article and give examples like (the) Italians, (the) Mexicans, (the) Turks and so on. From that we can understand that "the" is optional.
When do we use the Italians and Italians (without article)?
Is it considered to be one of the half-general cases?

Dear teacher, as a learner of EFL, I don't quite understand the nuance and distinction between them. Could you please explain it throughly?

Many thanks.