If you're playing pagan get your retinues up and have them going into the enemy's territory and then declare war like a day or two before they enter the county. I normally win before anyone else gets close.
Good strategy! Let me add using your tributaries if you have them. With a small start, I usually have 8~9k tributaries army when I only have 1~2k levy left out of my 5k cap, and less than 1k retinue.
It is much more powerful to make most of your neighboring chiefs and petty kings as your tributaries than annexing them. I would only use a few border dispute or county conquest CBs so I can reach more neighbors for tributary CB for benefits including;
1. Newly annexed your realm counties suffer severe penalty while your new tributaries' levy will replenish much faster.
2. Your vassals can refuse to join your wars while your tributaries can't.
3. Adding tributaries doesn't increase threat level, and you can annex them much later when your threat level is already at 100%.
4. Your tributaries won't mind if you become a bit tyrannical to your subjects, and be forced to join you if your own vassals ever revolt.
5. Tributary CB are free to call and you get 200 prestige from winning one, which you can use to call other CBs that cost heavy prestige - such as force vassalization CB at 250 for a count and 1000 for a duke.
Now you have a huge tributary allies. To use their army as your pseudo retinue, first declare war to a small enemy and call all your tributaries and vassals to the war. Wait till all their army is attached to your tiny retinue. Spread out your 150 retinue units so they can have several rally points. While waiting for that, after your allied armies started to move to your retinue units, you can raise and send your levy to actually fight that small war.
This strategy makes the new prestige retinue system as good or even better than the old 2500 a pop tribal event army, even when you have only one retinue unit with 150 men. If you have to enter peace for whatever reasons, your prestige retinue stays and your ally army can be called and attached again, where your event army would be gone forever.
Without ending the small war, disband your levy and march now 10k stack to your main enemy. Or 2 5k stacks. You can't separate attached armies into smaller groups. But they initially attached your multiple retinue units and you don't have to combine them.
When your 10k stack is one day away from crossing border to your main enemy, a large pagan kingdom for subjugation CB maybe, finally declare war to him, and call all your tributaries and vassals to the new war. Wait a week or two until they all answer the call. Now their armies will remain attached to your tiny retinue even if you finish the first war which actually called them. By keeping an unended small war somewhere, you can keep all your tributary armies as if they are your own retinue.
By the time your own retinue got bigger and your levy replenished from it's initial wars which created all those tributaries, your tributary army must have suffered a lot of battles and became pretty smaller. Especially if you stormed, as you should, those last few castles instead of slowly sieging them because those are mostly your allies' men and not yours.
What you got from them is all they had, because unlike feudal vassal levy, your allies send every men they have when called. So they got quite weak while you got stronger. The last major subjugation war made you a King. Your once ducal tributaries have served you for 5 years which was length of the truce, and are ready to be released and be forced to be your vassal for a 1000 prestige CB each.
It is time to turncoat. Do it one petty king at a time and fight with your levy helped by your still remaining tributaries, starting with the strongest. When your main war was near end, you could have called another small war, calling everyone except your strongest tributary, and end the first war. One the same day the main war is one, that tributary's army part of your stack will still be there for that day unless you exit the game.
If you release tributary and call war to it immediately, it will turn hostile and attack the rest of your allies, which is bigger (if not you should have already sent more retinue and even mercs, but not own levy) and may enjoy defensive terrain bonus, winning the decisive battle quickly to substantially raise your war score immediately. From your home, now you can call your rested levy since the betrayal war is already declared, and march it to his undefended capital.
It's cowardice. But if your largest tributary is a blobbed huge duke that you could only win the war by attacking it while it was fighting a major war of its own, which made his realm several times larger than your realm, you need all it takes to bring stability to your new kingdom. Young Wolf wouldn't have been murdered with his mother, wife, and unborn child if he was smarter. As good as the strong tributary is for now, it will be automatically released with your character's death.
There is no tyranny or revolt with tributaries. When the ducal tributaries are all vassalized by force, demand your count tributaries to be vassalized peacefully. Most of them will accept within a month if not immediately. The left will still be your tributaries and forced to send 100% of their levy whenever called to a war. For this, you don't need to release them first at 200 prestige each.
