Wikipedia articles are written in summary style. This guideline underscores how articles should split as they become longer.
Articles begin with a brief summary of the article's contents, followed by sections that go into detail. But as those sections get longer and more detailed, Wikipedia editors will create new articles that deal with those smaller subjects all on their own.
The example this essay uses is World War II. The article contains a summary of the background of World War II, but it also includes a wikilink to a separate article called Causes of World War II, which goes into much greater detail. This follows with nearly every section of the World War II entry. These "child" articles are encyclopedia entries on their own. They contain their own introductions, sections and references. As with parent articles, a topic must be notable to warrant a child article.