One of the best Intel missions is to get your neighbours to view another country as a threat to them. That will help a lot in keeping them from entering a faction or joining an alliance. As Germany, for example, keep threat missions going against France, the UK, and the USSR so that other countries become less inclined to join the Allies or the Cominterm.
You can see the video tutorial on Intelligence here: [http://www.youtube.com/watch?v=NYtMirGJIOM&list=PL177CF59636F80BD7&index=7 Hearts of Iron 3 FTM - Basic Tutorial - Episode 7 - Intelligence]
== Military Basics ==