Most of the major Wikipeida articles were written in 2006 and 2007, and "and have gotten relatively little attention from editors since then."

"After an encyclopedia reaches 100,000 articles, the pool of good material shrinks. By the time one million articles are written, it must tax ingenuity to think of something new. Wikipedia," Jensen writes, "passed the four-million-article mark in summer 2012."

With the exciting work over, editors are losing interest. In the spring of 2012, 3,300 editors contributed more than 100 edits per month each -- that's a 31 percent drop from spring of 2007, when that number was 4,800.

Taking a close look at the Wikipedia article for the War of 1812 (of which he is the most active editor), Jensen sees a pretty steep decline in editing activity. The article, painstakingly cobbled together by 3,000 editors, runs 14,000 words. The back-end discussion page where the volunteers worked out disagreements (you know, little things, such as who won the war) had 600 participants and runs 200,000 words.

Today, the War of 1812 page has many more readers than it did in 2008 -- 623,000 compared with 434,000 -- but the number who make a change has dropped precipitously, from 256 to just 28. Of those original 256, just one remains active. The reason, Jensen believes, is that the article already has had so many edits, there is just not that much to do.

The article for World War II, which Jensen calls "the most important article on military history," has experienced a similar phenomenon.

"The Wikimedia Foundation has an education program to turn undergraduates into editors. It may be easier to turn them into history professors," Jensen muses.

Obviously, the completion (or near-completion) of so monumental a task is an achievement to be celebrated. The problem for Wikipedia is that just because it has reached this point, that doesn't mean it no longer needs editors. A dedicated cadre of editors is what prevents vandalism and errors -- both intentional and innocent -- from creeping into its pages. But Wikipedia will have a hard time getting and maintaining that editorial vigilance if there is less substantive work for volunteers to do. Scrubbing out vandalism on its own does not make for interesting, long-term work.

Jensen believes that there is a way out of this: "Wikipedia is now a mature reference work with a stable organizational structure and a well-established reputation. The problem is that it is not mature in a scholarly sense." Wikipedia should devote more resources toward getting editors access to higher-quality scholarship (in private databases like JSTOR), admission to military-history conferences, and maybe even training in the field of historiography, so that they could bring the articles up to a more polished, professional standard.

H/t @yappelbaum