Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
As mentioned earlier, players can only declare war after meeting certain conditions like not having a non-aggression pact, high world tension, etc. However, the easiest way to declare war is to change to a communist or fascist state.
Source: https://gamerant.com/hearts-of-iron-4-how-declare-war-guide/