In the multiverse of Dungeons & Dragons players are confronted with a number of realities that are nothing like modern life, regardless of where you reside. The biggest are things like magic and roaming monsters that have a tendency to flip plans and schemes upside down with reckless abandon. There are a number of smaller realities to cope with as well, though. Things we take for granted now that wouldn’t exist in the setting of the game.
One of those things is how nations or even powerful individuals with their own armies set about starting or ending a fight. Most civilized states send ambassadors to speak on behalf of their superiors to prevent little grievances from developing into great big feuds. Trade and commerce can be a source of contention naturally. And then there is always the warlord that gets a burr under his saddle to pick a fight with an established monarch for whatever reason they deem worthy and all-out war ensues. What are the processes? Does a King need permission before attacking a neighbor?
The short answers are that there isn’t much of a process and no, a King doesn’t need permission to invade a rival’s territory. Who would they ask permission of? In some cases, such as on the Forkanzan peninsula, there is an overarching authority over a lot of things in the form of the Holy Imperium Church. The Supreme Pontiff of this church can decree that one or another of the city-states in Forkanza is justified in their invasion of another city-state due to reasons specified by the Church. This kind of “Clerical Sanction” gives one ruler permission to attack or defend accordingly. However there isn’t much to prevent them from more covert forms of warfare. And this only applies to those that accept the dominion of the Holy Imperium Church.
In other parts of Partum there are several states that have alliances with each other specifying various degrees of cooperation or at least spelling out means to avoid open conflict with one another. The Council of Schönbrunn is one such example. If the King of Fanolania decides that he has had enough of those accursed orcs that govern Oublier, though, then the orcs are pretty much on their own to prevent the Fanolanians from running them into the ground. They might have alliances of their own they can call upon to help them resist a fight with Fanolania or commit resources to defend against such an attack should it begin. Nearby states not affiliated with the orcs could be alarmed by such a move by Fanolania, but there isn’t much of a process for these states to do much other than gear for war themselves.
This balancing act keeps any one country from becoming too much of a threat to everyone else’s security so long as nobody has a burning desire to just conquer everyone else and consequences be damned. Such a move would mean that a state has truly invested in nothing short of a massive military endeavor because anyone that resists will be targeted for attack as well. Medieval states were simply not designed for this level of “take ’em all down” strategies. This was more the method of operation for the Third Imperium and look how that turned out. Wars as a result tend to be fairly short and with limited goals to reach. Using marriages and other forms of non-military means to reach amicable unions were and are far more likely to get the results you want without risking getting yourself killed. Another fine idea is to send in a small group of adventurers to settle the score before things get too heated. If they fail, you can then try something more decidedly aggressive.
So all a King really needs to do is declare a war and then see it through. He will have to authorize the formation of an army, equip and lead them forward to battle. Or he can order somebody else to do it. Either way there aren’t many options in terms of checks and balances. Its good to be the King.