外交

For the most part, Diplomacy in HoI4 is bilateral, meaning all relations, offers and requests are done one to one, between the player and the target country or between two AI countries. The exception to this is when a faction is created. All members of a faction will join defensive wars together and may also invite other faction members to join offensive wars along with the aggressor.

Additionally, diplomacy is not symmetric. This means that all variables affecting the opinion of another country or the likelihood to accept an offer are completely independent for each country. For example, the player country may have positive relations with another country while that country might have negative relations with the player. Relations can be increased by being in the same faction, having the same Ideology, by Guaranteeing, through trading or improving relations.

Diplomatic actions

There are a number of diplomatic actions in the game. All the diplomatic actions in the following table are the primary action, all the actions that are cancellable have a related diplomatic action that becomes available once the primary action is taken. It will also replace the primary action in the diplomatic menu. The only exceptions are Expeditionary Force, which will be cancelled by "Return Expeditions", and Ask for control of State, which will be cancelled by "Give control of State".

Action	Description	Requirements	Conditions to cancel
Declare war	Only way to start a new war.	War goal on the target country.	War has been ongoing for more than 6 months. Must be faction leader. Must be on the losing side. The war score of the winning side is at least 50%.
Justify War goal	Begin justifying a war goal against another country.	Target country must not be an ally. Variable amount of . Amount decreases as  World Tension rises. Amount is also modified by Subversive activities modifiers. Minimum of 2.	None
Guarantee independence	Guarantee the independence of another country. If anyone declares war on a guaranteed nation, that guaranteed nation will automatically join the faction of the guarantor.	Target country has the same ideology. Costs 25 +25 for each country already guaranteed.  World Tension threshold reached.	None
Call to arms	Calls an ally into a war.	Is a member of the same faction.	Cannot be cancelled.
Ask for military access	Military access allows moving troops through the territory of another country, allied or not. Military access is granted automatically to members of the same faction.	Target country must accept.	No troops on the target country's territory.
Offer military access	Offer to provide military access to another country. If they accept, it will not automatically grant you military access.	Target country must accept.	The target country has no troops on the territory of the country that has been granting access.
Ask for docking rights	Docking rights allow moving ships into the ports of another country, allied or not. Docking rights are granted automatically to members of the same faction.	Target country must accept.	None.
Offer docking rights	Offer to provide docking rights to another country. If they accept, it will not automatically grant you docking rights.	Target country must accept.	None.
Improve relations	Start improving relations between you and the target country up to +100 over time. Relation affects trade influence and the success likelihood of other diplomatic actions.	Must not be at war with target country. Costs 10 +0.2/day if both countries have the same ideology and +0.4/day with differing ideology.	None
Send Attaché	Sends an attache to a country. This gives the sender additional 陆军经验 and 战争支持度. They can also see the receiver's military positioning.	DLC required. Target must be at war. 100 to send the attache. 50 command power is used to maintain the attache.	None.
Non-aggression pact	Having a non-aggression pact (NAP) disables "Justify War goal" and "Declare war" diplomatic actions.	Target country must accept.	Not cancellable without having a common border or within the first 12 months of the pact. After 12 months, the ability to break a NAP will depend on the relative size of armies along the common border. Up to 18 months past the NAP, the country will require a 2:1 ratio of troops compared to its opponent on the common border. Between 18 and 24 months, a 1:1 ratio is required. Past 24 months after the NAP, a 1:2 ratio is required to break the pact. NAPs dissolve after four years.
Invite to faction	Invite another country to your faction.	Target country must accept Requires popularity of inviting country's ideology > 30%	Must not be faction leader. Must not be in war with other faction members. Three months must pass between a country joining a faction and them being kicked.
Request License Production	Licenses equipment from target country, allowing you to produce target equipment and research target technology faster. Civilian factories are used to pay for license.	Target country must accept. 玉碎瓦全 is required.	Two months minimum must pass before a license can be cancelled.
Create faction	If you are not already in a faction and a National focus allows you to create a faction, you may invite another country to create a new faction.	Same ideology. Target country must accept.	Must not be faction leader. Must not be in war with other faction members.
Join faction	Allows you to join a faction. Will automatically join existing war(s).	Target country must be faction leader. Must not be in a faction.  World Tension threshold reached. Faction leader must accept.	Must not be in war with other faction members. May not Leave Faction if already in a war as part of the faction.
Start Lend lease	Send any equipment to another non-enemy country currently at war. It's possible to send a single shipment of a specified amount, or set a specified monthly amount or percentage of monthly production to be sent. Shipments occur once per month and information on past and present shipments is available in a tooltip for the Lend-Lease section at the top of the Recruit/Deploy window.	Target country must have enough convoys to receive the equipment, unless there's an unobstructed land path.  World Tension threshold reached.	None. Click on the Lend Lease action to modify or cancel Lend Lease instructions.
Request lend lease	Ask a country to send lend-lease of specific equipment archetypes. The selection screen shows the stockpile and production rates of the target country.	Requires 共赴胜利. Target country must accept. Must be at war.	None
Join wars	Allows you to join the war of another faction member.	Must not already be in war with allies. Target country must be faction leader.	Not cancellable
Send Volunteers	Send a limited number of divisions to a country. Control over the units is retained, but return only when donor is at war.	Target country must be at war. Must be at peace. Must not be in the same faction as the target country. Total number of divisions threshold reached.  World Tension threshold reached.	None
Expeditionary Force	Send an unlimited number of divisions to a fellow member of the faction. Control over the units is transferred.	Must be fighting a war together. Target country must be an ally. Target country must accept. Divisions must be on land.	Donor must activate Return Expeditions to recover Divisions. A target country Disband order to Expeditions returns them to donor.
Return Expeditions	Recovers control over all units currently sent as Expeditionary force to the target country.	None	None
Request forces	Gives control over a selected number of divisions that are part of the military of the target country.	Target country must be our puppet.	A Disband order to the requested forces returns them to the puppet.
Boost party popularity	Increases the popularity of the target country's local party with the same ideology. With La Resistance enabled, this diplomatic action is moved to a mission an operative can do.	Costs 0.25 per day	None
Stage a coup	Starts preparations for a Coup d'État in the target country. The most common effect is to start a civil war in the country. The result of this action will depend on the popularity of the political party belonging to your ideology. With La Resistance enabled, this diplomatic action is moved to an operation.	200 or more, accumulating at a maximum rate of 0.50 per day. This cost is modified by Subversive Activities modifiers. Thousands of infantry equipment.	None
Ask for control of state	Ask current controller of an occupied State to give up control.	Must be in a war together. Must have sufficient war contribution. Target country must accept. Target State must not be considered a Core of the target country.	Can (in theory) be canceled through the "Give control of state" action. Otherwise not cancellable.
Give control of state	Give control of one or multiple occupied States.	Target country must accept.	Can (in theory) be canceled through the "Ask for control of state" action. Otherwise not cancellable.
Reduce subject autonomy	Reduces a subject's autonomy level.	The subject's autonomy is low enough to be reduced a level. Can afford the 政治点数 cost.	Cannot be cancelled.
Request Garrison support	Ask a puppet or faction member to provide manpower to fill garrisons.	Target country must accept.	None.

Additional information

• Military Access: You don't need to give another country access to your own territory if both you and the second country are at war with a common enemy. For example, if the USA, UK and Soviet Union are all at war with Germany, then the Soviet Union will have access to UK and US territory to attack/defend against Germany while the UK and USA will have access to any territory controlled by the Soviet Union. This automatically happens when warring nations gain common enemies and cannot be prevented in any way.
• Guarantee independence: The  political power cost will start at 25 for the first guarantee and will then increase by 25 for every subsequent guarantee, in the following fashion: 50 for the second, 75 for the third, 100 for the fourth and so on.
• Join faction and Create faction: the conditions to cancel these diplomatic actions are essentially the same. They're the conditions to activate the Leave faction diplomatic action, except that you cannot be the faction leader when you join a faction. The tooltip does not warn you, but if you go to war along with the faction then you may not leave it, so Leave Faction is timely in the event of a Call to Arms.
• Start Lend Lease: the amount of production that can be sent to the target country can be a fixed number of units out of the monthly production or a percentage of the monthly production, but not both at the same time. It's possible to modify the Lend lease at will. For further discussion of Lend Lease, refer to the Equipment section.
• Stage a Coup: This diplomatic action also requires to select the State of origin of the civil war. At the onset of the civil war, the rebelling State will be joined by a number of other States, the number of which will vary based on the proportional party popularity when the full political power requirement is filled. For this reason, Stage a Coup is more effective when combined with "Boost party popularity" to well over 50%. The amount of infantry equipment required appears to be a function of the target country's civilian population. The more populous the country, the more infantry equipment a nation has to give in preparation for the civil war. Coups will automatically succeed with over 40% party popularity if performed in a country with only one state. Otherwise a successful coup will start a civil war. For further information about civil wars, refer to the Civil War article.

World tension

主条目：World tension

 World Tension acts as a measurement of how close the game is to become a world war. Many diplomatic actions are limited by world tension. Annexing countries, joining factions, declaring war on nations are all examples of things that increase world tension.

Generally speaking, Axis powers want the world tension to stay low so they do not get a massive coalition against them, while democracies like the USA may want world tension to increase so that they can intervene.

When world tension is high, a country that has war declared upon it is likely to accept an offer to join from a faction of the same ideology.

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