I. Weak claimants can only press their claim under specific circumstances

• Regency
• Male claimant against female ruler
• There is already a war in progress disputing the title
• Claimant is currently a pretender (2nd or 3rd in line to inherit title)

II. Females cannot press claims against titles with agnatic succession laws.

So, when you see a claim that is not available as a CB

1. If the claimant is female, verify that the title succession law is agnatic-cognatic or absolute cognatic
2. If the claim is weak, verify that at least one of the specific circumstances apply
3. Male claimants with strong claims can always press their claims

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