Casus Belli

A Casus Belli is a justified reason of war. If you do not have a Casus Belli before declaring war on a nation, neutral nations can declare war on you with the 'Anti-Aggression' Casus Belli. For this reason, you should usually have allies to back you up if you wish to make an Aggression Expansion. It is worth to mention Nether Colonies are exempt from all Casus Bellis, meaning the colonies can only be at war if their parent nations are.

Self-Defence
All Government Types can use the 'Self-Defence' Casus Belli. It is exactly what it sounds like. If a nation declares war on you, then of course you can defend yourself. There are other events that qualify as self-defense too. They include:
 * Being Raided. However, if the faction leader ousts the raiders, you cannot declare war. If you oust a raider, you cannot allow them back into the faction unless the other faction agrees. The Piracy government type is an exception, so please check out the page for information.
 * Attacking an ally, puppet state, or colony. If another nation declares war on your vassal, or a nation you have interest in protecting, you have a Casus Belli to defend them.
 * Spy Detected. If a spy from another nation is discovered, you can declare war on the faction if you have enough evedence, even if the faction leader ousts the spy.

Anti-Aggression
All Government Types can use the 'Anti-Aggression' Casus Belli. If one nation declares war on another without a Casus Belli, you may attack the attacking nation, but only if you, or a colony is within 1000 blocks of the attacking or defending faction.

Imperial Liberation
Most governments can use the 'Imperial Liberation' Casus Belli. If a citizen is in a faction without their consent, and they admit that to you, you can declare war on the faction, if they don't willingly send the citizen to your faction. If you win the war against the Imperial faction, you must admit the player into your faction.

Reconquest
Federative, Oligarchy, Monarchy, Piracy and Autocratic governments can use the 'Reconquest' Casus Belli. If a nation defeated you and took your land, you may go to war to take it back, if it less than a month from the previous war. If you use the reconquest Casus Belli, you can only annex the region you lost.

Ideological Expansion
Communist and Autocratic governments can use the 'Ideological Expansion' Casus Belli. These 2 government types are the only ones to use this Casus Belli, yet they can use it against each other whenever they wish. For example. a Communist government can use this Casus Belli against a Authoritarian government, and vice versa.

Border Friction
Most governments, aside Anarchism, can use the 'Border Friction' Casus Belli. If 2 nations constantly reclaim land of each others, and not an official border has been set, this can be used as a declaration. However, do keep in mind that neutral nations can declare war on you or the opposing nation during the war if this Casus Belli is used, so please be careful.