White peace with the specified countries. Print out all console commands to game.log file. By now you have probably figured out that nukes in HOI4 are not an easy and quick solution to handling conflicts. Does not work if you put another nations tag in the command such as 'set_country_flag flag AUS'.
An easter egg making the AI smacktalk in chats. Sir, yes sir! (Can declare war without justification), This is likely the most effective way of wanting to start a war.