I just started a new campaign as the Prussians in the un-modded base version of E:TW. It's taken me a while to learn this trick (I don't usually bother to get fancy with diplomacy), but oh, the havoc you can wreak when you get yourself military access to your neighbors...
Less than a decade into the campaign, Austria simultaneously declared war on me and France (no idea why, we weren't even friendly, let alone allied). I held them off and took Silesia, while the French couldn't stop them and lost Alsace-Lorraine. Since that's one of the territories I need for a victory, I snuck an army through Hannover and the Rhineland and took it from them. The ensuing attempts of them and their allies to get it back from me ended up getting me an army that was at level 5 experience all across the board, and about six armies destroyed (not all massive or well trained, of course)... all by 1720. I now have an extremely well trained army in an easily defended province, poised to strike almost anywhere in western Europe in two turns or less.... oh my.
It's good to be German and Evil.
