Playing as Germany, I was at war with, Russia, Norway, Sweden in the north, Slovakia, Hungary, Albania, Greece, Bulgaria and Romainia in the south/south east.
All countries from the southern area's sent wave after wave of units though neighbouring territories to attack me. I had my attention elsewhere, and I was deep into the Netherlands, Belgium, and had just annexed Luxembourg. So I had to do some quick thinking to push them back after an initial success in pushing into German lands.
Thank god I let my minister control garrisson troops.

Was epic fun!
On the sub thing. The other day was the first time ever, Ive built the converted Carrier to load up anti-sub aircraft, and actively partol the Skaggerak area for Russian Subs. Having to actively take anti-sub patrols is a nice feature.
Although I wonder why most anti-sub aircraft cannot spot them. Kinda seems to defeat the object concidering it IS a recon plane anyways. Surely it should have some even if limited precision spotting range?
Anyhoo. What I found fun but a little annoying was relesing a carrier from Kiel to make a anti-sub battle group, only to find 3 swedish subs sat right outside the sea pier, and insta popped my carrier. w00ty!
Now all carrier construction is done in Germanys inland river systems, and has the mouth to the ocean blockaded by my naval forces.
Im finding a battlegroup mix of the Elizabeth carrier from the uk with 3 squardons of intercepter aircraft aboard. A second but smaller carrier group for anti-sub patrols and a third carrier group containing the Necron UAV strategic bomber. Makes a nice mix of offence and defensive roles for a single area.
Sub warefare is fun and very unexpected. Nice one!