The principal database for identifying journal articles in American history is America: History and Life:
Although America: History and Life is considered the most "important" article index for research in American history, there are several other very important article indexes, any of which might be crucial for research depending on the focus of your research:
Below is a list of major, full-text periodical collections. Note that these sources are not comprehensive: many full text journals are only available, entirely or in part, through their publisher's websites. For example: The Sixties: a Journal of History, Politics, and Culture, Radical History Review, and Journal of Black Studies.
Journals like The Sixties, Radical History Review, and Journal of Black Studies, are indexed by America: History and Life.