The Aeneid cannot be understood as a historical book, and Gibbon, even if it's an excelent achievement in modern historiography, has been widely outdated. According to the modern theories of indoeuropean migrations, Romans came from the Italic indoeuropeans.
There are also two cultures which scholars use to connect with the indoeuropean migrations: the Terramare and the Villanovan culture. Some scholars believe in the unity of these cultures (V. G. Childe for instance), and some do not.
When discussing the origins of Roman people lots of problems appear, basically because of the unity of these Italic people. I'll give you one of the interpretations, that of G. Devoto: the indoeuropean people who went down to Italy are infinite in its variety, but we can differentiate three groups: one in the Padan plain, one in Puglia and the last one in the Appennine Umbro-Marchigiano. These three focus are caracterized for the treatment of the ide. aspirations: for example, you have the ide. root *rudh- (red), which derives into ruber (Padan plain), rutilius (Puglia) and rufus (the Apennine). Note that this division is based on the treatment of the voiced aspirate: b/t/f.
The Puglia culture was probably the oldest ide. culture in Italy, to which also belong other archaisms such as rex. The other ones came in little migrations and invasions, not necessarily with violence, and surely without any political system, and they were gradually put together, both politically and culturally, by the Roman people -which should be understood as some "mix" of them, culturally speaking.