Harbrace rule 14a: Use a semicolon to connect independent clauses not linked by a coordinating conjuction (and, but, for, or, nor, so, yet).
Another fairly handy semicolon construction is the following:
Use a semi-colon before such 25-cent connecting words as "hence," "however," "moreover," "nevertheless," and "therefore" if they connect two complete thoughts and if they are the first word in the second.
Right: Duke is a good dog; however, he has a serious problem with his Milk Bone habit.
A comma follows these words if they cause a pause in the reading. You have to play it by ear. "Therefore," for example, often doesn't need a comma after it.
As in --
I think; therefore I am. (Cogito; ergo sum.)
Note: make sure to use commas around these words when they are used "parenthetically" in the middle of a sentence.
As in -- My fear for Duke, however, is that his Milk Bone addiction will affect his ability to love and work.