a person who speaks authoritatively but not convincingly on topics that they have no practical experience with, (especially applied to advocates of war)
This armchair general gets on the soap box every day now.
A person who sits in a chair behind a computer and leads people or at least attempts to, instead of doing it in person. Usually complains about government, organization, military, political, or religious events through blogs, forums, and social networking sites.
Forum administrator, "Wow, these people are really getting angry, they might start a rally for real soon."
Forum administrator2, "nah, they're just a bunch of armchair generals, they won't do anything if it means having to get up and move around."