I recently read John Poole's Terrorist Trail, where this subject is a primary focus. All reviews that I've read and experiences in Iraq back up much of what he writes about where terrorists and global insurgents come from and how they get from point A to Iraq, Afghanistan, Bosnia, Chechyna, Pakistan, etc. While I'm not sure if Hezbollah has as strong an influence in as many places as he claims, the book's great nonetheless (plus I don't have anything to refute his claims).