This is the list of databases that would be most likely to cover secondary scholarly sources. Of course, sometimes you can get lucky and a primary document has been republished in a scholarly source, but I wouldn't count on it.
As an archive of scholarly journals, this database includes a large set of history journals. Remember that it is an archive - articles published in 1950 may still be relevant, but they also may have been challenged/ made obsolete/ etc.
There are 2 primary databases for scholarly articles from Historians:
Historical Abstracts-& America History and Life
Historical Abstacts started indexing journals in 1955 and covers world history from 1450 to the present. America: History and Life started indexing in 1964 and covers prehistory to the present.
When searching, if you come across an article that is not in full text, click the Find It button.
Web of Science (formerly Arts and Humanities Citation Index)
This broad database can be searched by cited reference (in other words, who has been using the seminal work you've found) and because of it's scope may provide more articles that are relevant to your topic. This is also useful for ancient history.
There are many more databases that are available, but this is dependent on the area you are researching. For example, if you were interested in banking, you might look at business databases too.
OneSearch - Search for Articles, Books & More
Sometimes you may still need to use an old print index.
Poole's Index - 1815-1906 Call # AI 3. P66
Readers Guide to Periodical Literature 1900-1998