How to invade a whole kingdom in Crusader Kings 2?

I'm am playing as England. I wanted to invade Scotland and Wales. When I try to fabricate a claim I only get a claim on a county, not a whole kingdom. It takes a lot of time to capture a kingdom that way and when I try to fabricate a claim on the Capitol of the kingdom it doesn't work.

Is there a way to declare war on a whole kingdom?

There are a couple of things that make conquering a kingdom hard.

1. Getting a casus belli (cb).
2. The truce waiting period. (You can break a truce, but the penalty is harsh)

Ways to literally invade the entire kingdom at once

1. As mentioned by svick, if you are an emperor, you can press a claim on the kingdom title for a relative or vassal, and upon winning, the claimant will be your vassal.
2. If you are invading realm that is larger (more holdings) than yours, you can use the Invasion CB
3. If the target king is a heretic of your religion, your religious head might declare a crusade against the king.
4. If you have a claim on the kingdom, you can press your claim.
5. If you are playing as a pagan (requires The Old Gods expansion), the become king of [kingdom] ambition gives you a CB against realms inside the de jure territory of the kingdom.

Ways to use Love, not War to gain a kingdom

Beware, an untimely death can spoil your plans. On the other hand, you may be able to use assassination or a plot to arrange a timely death.

1. Position a future heir of your kingdom to inherit the target kingdom.
   • The most direct way: If the target kingdom is ruled by an unmarried queen, your male heir (or you, if you are male and do not have a male heir) can marry the queen. Assuming a child is born and compatible succession laws, your heir will rule both kingdoms.
2. As mentioned by svick, position a future heir of your kingdom to inherit a claim on the target kingdom. For example, have your male heir (or you, if you are male and do not have a male heir) marry a princess with an inheritable claim.

With claims, you may have a choice between

1. Pressing the claim of your heir's parent, setting up your heir to inherit both kingdoms
2. Waiting until your heir inherits both your kingdom and the claim on your target kingdom, and can press the claim playing as your heir.

Dividing and conquering

1. If vassals of the target king are rebelling, you can declare war on a rebel. You still need to find a CB. The significance is that you can declare war on rebelling vassals without penalty even if you have a truce against the liege (king, in this example)
2. Especially if you cannot marry into a claim on the kingdom, consider pressing claims on duchy titles in the target kingdom.
3. Once you gain more than 50% of the de jure counties in the target kingdom, you can usurp the kingdom title. This tends to make conquering the rest of the kingdom easier because
   • The remnants of the old kingdom will probably be fractured into different realms
   • You get a de jure CB on the rest of the de jure counties
   • Rules in the de jure realm, especially counts, may agree to become your vassal without a fight

As far as I know, there is no quick and easy way to do this. You have several options:

1. Invade county after county, until you have enough counties to usurp the title.
2. • Find someone who has an inheritable claim to the kingdom.
   • Marry them, have a son
   • Wait until you die and become that son.
   • Now you have a claim to the kingdom and you can invade it. (If it's a weak claim, you will have to wait for the right circumstances.)
3. If you're already an emperor, invite someone with a claim to the kingdom to your court, grant him land and press their claim. This will make the kingdom part of your empire, but not personally yours.

I have found it best to not press claims immediately upon fabricating them. You can fabricate claims on multiple counties in a kingdom, and then when you declare war on the king, you can press all claims at once. You just have to balance that with the likelihood of your ruler dying, which would make you lose most, if not all, of your claims. The benefit of going this route is that you don't have to face the resources of an entire kingdom and its allies multiple times at a gain of one county per war. You also don't have to wait for truces to expire one county at a time.

When you start getting multiple kingdoms, you quickly learn that keeping your kingdoms is actually a lot harder than gaining them. I prefer elective monarchy, so you can have a larger demesne than primogeniture, and you can select the best member of your family to be your next playable character. This gets tricky when you have to rig multiple elections, though. An untimely illness or assassination before you can get your ducks in a row could be devastating. A nice trick is that if your preferred heir doesn't look like he has a chance of being elected in one kingdom, you could just grant the kingdom to him as a landed title before you die. The biggest threat here would be your king outliving the preferred heir, which would trigger another election in the kingdom that is more or less outside of your control.

If you want more than one kingdom, you need to become emperor as soon as possible. That way, even if you lose a kingdom, the new king will still be your vassal and the kingdom will still be in your realm.