So the year is 1230. I'm the Empire of Alba, and started as a count in Dublin. Currently I control Ireland, Wales, England, Brittany, a small section of Scotland (which I took from Norway), and a bit more of France near Brittany. The HRE is getting ridiculously huge and now they're right next to my land, and HRE dukes keep trying to forge claims on my stuff.
Screenshot of map of mostly western europe, on steam.
They were my only ally, but then they declared war on "Castille," which held the only remaining non-HRE French territories left, besides my own, which were all just east of mine. Not wanting to lose them, I set a claimant up as King of Wales and pressed his claim on Castille myself. This put us in a state of hostilities, and our armies clashed, and we both lost a lot of troops. In the end, they brought in an army about 70% the size of mine, which had the remarkable ability to snatch victory from the jaws of defeat. I lost against that army in each battle and in the second or third one they eradicated my entire army. I brought in reinforcements, and tried to resume taking Castillian stuff, but it was too late: The HRE had gotten ahead of me, and they hit 100% soon after, and took all of Castille.
There's no war between us now, but I don't see them listed in my alliances anymore either, and they've still got more troops than me and there's no more buffer zone. Either I need to eat someone else, or ally with someone else, or I need a way to break up the HRE into bite-sized chunks... Of course, I may be glad they're this huge if the khans become a problem in the future. The Ilkhanate already has territory over in Persia.
Castille, by the way, was ruled by the King of England when I invaded England some time ago. After that, Castille was all that was left - if you include his territories in France, which were larger and more continuous than his territories in Spain, but apparently not kingdomy enough.
I went from Ireland to Brittany and siezed most of France with holy wars (Note to self: Don't be a heretic in CK2, ever, well, unless you're the HRE with 50,000 troops), then I acquired most of wales, and then I got the Pope to authorize an invasion of England because reasons, ho ho ho. Alas, I won rather quickly and failed to remove the english nobility, so they were not amused at being conquered, and when I formed the Empire, they attempted to vote me off the island (or at least attempted to vote for an english heir).
Sadly, the Pope excommunicated me during the invasion of England (at the request of one of my Welsh vassals, the jerk). I solved that problem by switching to free investiture in Brittany - I'd already moved my primary title to Brittany and my capital into France (Poiters?) - and then appointing an antipope. I later accidentally "forcibly deposed" my antipope when I gave the province he was in to someone, and since the real pope still hated my guts, I had him assassinated. The next pope liked me a lot better. Of course, now I think there's another pope and I have another emperor entirely (and this emperor has mostly been concerned with trying to rein in the english nobility, and by that I mean taking titles away from dukes when they are dumb enough to do a plot and get caught at it). I also lost the english throne to the family idiot because it was stuck on hereditaryness (and still is) instead of elective, and had to provoke him into rebellion to get it back. This was still some time ago. I'm beginning to wonder if maybe I should have only made one kingdom, and no empire. It would probably have been simpler to manage. Not sure if I'd be able to join the HRE and fight other vassals or not if I did that, though. Right now my only option is to fight the entire thing, as far as I can tell.