Plenty of professors teach online now.
Er, that is, plenty of professors have scanned an armload of articles into PDF and upload them to Blackboard or some other learning management system, and have students post a few times to a "discussion" board. You will note that there is no teaching involved in that exercise.
Let's assume you're after more than that, and that you want your students truly to discuss history as a part of the course requirements. How do you accomplish that? The answer is simply stated but difficult to execute: set clear requirements and stick with them, and model behavior. This post concerns the first part.
Sometimes when I tell a colleague that I require of my students three posts a week, the ol' eyebrow goes up. It's not said, but the thought is "only three"? Yep, only three. It's a matter of mathematics.
My expectation is that every student reads the posts of all the other students. If there is a class of thirty students, that means ninety messages a week (well, eighty-seven) to read. Add to this the posts from the teacher and the student is looking at 100 to 120 messages a week to read. No, I don't think they read all of them either. But they do read most!
Raise this to four messages a week and now you're at 150 or more messages a week. Cut it to two and it drops to eighty or so a week. Now, the real numbers are less because you can always count on the under-performers and those who drop the course. The point here is that the requirement is very much driven by how much it is reasonable to expect your students to read. If your class is larger, posts per week can go down; if smaller, the figure can increase. To put it another way, the requirement is driven not so much by what it is reasonable to expect the students to post, it's driven by what is reasonable to expect them to read.
Beyond this we have matters of form and content and other considerations. I'll leave those for another entry.