A Defensive pact is a multilateral agreement to defend against an extremely threatening neighbor. If the subject of the pact attacks one of the members, others will join the war like allies.
Membership in defensive pacts is influenced by threat, a value for each realm that increases during expansion and decays slowly over time.
Defensive pacts can be disabled in the game rules before the start of the game session.
Threat
Increase
Each county conquered adds between 2% and 15% threat. The exact amount depends mostly on your realm size(?). Claim and de jure wars generate less threat. Diplomatically vassalizing other rulers, or personally inheriting titles when you are already a ruler, also increases threat, but at a lower rate than war. A single source can add at most 50% threat.
If your vassals generate threat, some of this threat will be added to your own. This effect is much smaller than the effect of expanding on your own, since any given vassal is too small to generate much threat per county.
Decrease
Threat decays at a rate between 0.21% and 0.4% per month. The exact amount depends on your military strength(?). With 
Monks and Mystics DLC, your Chancellor can be assigned to a job which increases threat decay.
Any land leaving your realm can reduce threat; this includes land lost via inheritance and conquest.
Granting independence to vassals decreases threat based on the number of counties freed. This encourages you to release outlying vassals, especially those who provide fewer levies due to de jure structure. Releasing your furthest vassals can also shrink defensive pacts, because fewer foreign realms will be close to your borders.
It is best if you already have a non-aggression pact with the vassal you plan to release, so they can't join defensive pacts; this is more important if the vassal being released is not of your religion. In addition, you can request an alliance.
Threat levels
At low levels of threat, defensive pact members are organized by religion group. As threat increases, even infidels will fight together when defending against you.
| Threat | Effect |
|---|---|
| 5% | Minimum for defensive pacts against you |
| 50% | Religious groups other than yours will band together |
| 75% | All religious groups band together |
| 95% | Even your religious head and holy orders will join all wars |
Threat caps at 100%. Thus, any threat beyond 100% will be lost.
Membership
- In my ERE game, I see fellow Christians only joining defensive pacts when I have a CB, and even then not always. On the other hand, Pagans far too distant for holy wars are all in the pact. This isn't fully explained by what I gathered below, from reading defines.lua. What am I missing?
Any realm can join a defensive pact against you if:
- Your threat is at least 5%
- They are smaller
- They are within 300 distance, or 400 distance if another religion group
AI will consider joining against you if:
- You could attack me: Less than 80% troop strength and you have a CB (except "Make tributary" or "Humiliate nomad"?)
- You're too big: Less than 20% your size in provinces
- Pact needs assistance: pact is less than 70% strength, an existing member of their religion/culture/dynasty, and you are not
AI will then compute a score to decide whether to actually join:
[(1.0 × provinces) + (150 × threat) + (-1.1 × distance)] * cb_multiplier
They will join with a score above 110 and leave with a score below 70.
Strategies
There are many things you can do other than simply waiting for threat to decay.
Go all in
When fighting a war that would take you from 40% threat to 90% threat, declare as many other wars as you can handle at the same time. This is your last chance to attack moderately weak realms until (1) you can take on entire regions at once or (2) your threat decays.
With the 
Conclave DLC, if your council can vote on war declarations, be sure to have Glory Hounds voting if you wish to pursue this strategy and not cause tyranny and council discontent.
Attack targets who are not in defensive pacts
- Attack new realms the day they are created, before they have a chance to join defensive pacts. Make sure you have a CB ready.
- Pick off vassals who are rebelling against their liege.
- If a realm has not yet joined a defensive pact, avoid gaining a CB on them until you're ready to attack. For example, only invite a claimant at the last minute.
- Invade realms that are stronger than you on paper, increasing your de facto power through alliances or by hiring mercenaries and holy orders.
Blitz small members
Pick off tiny defensive pact members by assaulting their holdings very quickly. If you completely occupy the target realm before the rest of the defensive pact shows up, you have 100% warscore and can declare the war over.
The main problem here is that levies cannot be raised when declaring war. Retinues or mercenaries, on the other hand, can be days away from arriving in the province. These professional armies are expensive, but there are many ways to gain wealth.
Another obstacle is the presence of upgraded castles; castles with fort level 6 and above cannot be assaulted; be sure to check the fort levels of castles before declaring war.
If a realm is slightly too large for a quick retinue assault, you can use a two step plan. First, find an excuse to be hostile to their armies and holdings. Use levies to weaken them as much as possible. Second, declare war on them and finish them off with retinues.
There are several ways to become hostile to a realm without incurring a truce or getting the defensive pact involved:
- Being allied to their enemy:
- Join an ally in a war where your target is on the other side
- Join a coreligionist in a holy war
- Join a rebelling vassal (
overthrow_ruler*rebellions only)
- Being at war with their rebelling vassal
- Raiding
Support vassals in their external wars
- Establish strong vassals near your borders, even if you normally prefer to have weak vassals.
- Raid neighbors and destroy their armies, so your vassals are more inclined to declare war.
- Alternatively, raid your vassals' targets as they fight their wars.
- Assist vassals with cash gifts, defeating their enemies' armies, or (depending on the CB) joining their war
Form non-aggression pacts
- It helps to have a large family.
- Use favors to arrange marriages with infidels. You probably won't be able to buy a favor from a ruler who is in a defensive pact against you, but you might be able to buy a favor from their heir for use later.
- You can even create a non-aggression pact with a betrothal, immediately break the betrothal, and attack. This is considered "gamey".
Use tributaries
- Making a realm your tributary forces them out of a defensive pact without increasing threat. It also increases your effective strength (since you can call tributaries to war) without allowing stronger realms to join the defensive pact.
- Good against empires or any other realm from which you can't take a significant fraction of land in a single war.
- If you do get a good CB against them later, you can release them from tributary status and immediately attack.
- Forcing tributaries to join your wars may weaken your tributaries enough to trigger revolts. You can then declare wars on the revolts freely.
- Use the CB for a tributary type which persists through succession.
Be a vassal
- When you conquer territory, any defensive treaty formed will be against your liege lord, who is perceived as the ultimate owner of this territory, and won't be triggered by future wars you initiate.
- This means that vassal
khans can expand with greater impunity than their Khagan.
Modding
Search defines.lua with the following regexp: infamy|threat|coalition|defensive_pact
See also
Other PDS grand strategy games have related mechanics:[1]
- Victoria 2 has Infamy. Internally, it is called BadBoy.
- Europa Universalis IV has a more nuanced version of threat called Aggressive Expansion, a relation modifier that indicates how threatened each nation is by your actions. When Coalitions form in EU4, they can act aggressively to dismantle a threat.