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.
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.||
|Justify War goal||Begin justifying a war goal against another country.||
|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.||
|Ask for military access||Military access allows moving troops or ships 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.|
|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.||
|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.|
|Invite to faction||Invite another country to your faction.||Target country must accept.||
|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.||
|Join faction||Allows you to join a faction. Will automatically join existing war(s).||
|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.||
||None. Click on the Lend Lease action to modify or cancel Lend Lease instructions.|
|Join wars||Allows you to join the war of another faction member.||
|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.||
|Expeditionary Force||Send an unlimited number of divisions to a fellow member of the faction. Control over the units is transferred.||
||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.||Costs 0.10 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.||None|
|Ask for control of state||Ask current controller of an occupied State to give up control.||
||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|
- 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 amount of states the join the rebellion will wary 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. In the current stage, it appears to be impossible to execute a successful Coup d'État, that is, one without a civil war involved. For further infomation about civil wars, refer to the Civil War article.
- Main article: 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.