Declaring War on someone is one of the few things in Diplomacy that does not require both sides to agree to a Treaty Proposal. As long as your Nation has at least one Level 10 Town, then anyone else on the world stage can declare War against you (unless you're a Vassal, or in an Armistice with the aggressor.)
Clicking the Declare War button in the Create Treaty Proposal menu will open a new menu showing the Nations you are about to drag into War. Remember - Alliances are tied together, so both the aggressor's allies and the defender's allies will be a part of the War effort.
To win a War, your side must gain enough War Score to have tipped the Balance of Power bar in your favor (viewable in the Nation Status menu) by the end of the 7 day period. War Score points can be obtained in multiple different ways - all of which break down into 3 major categories:
The first way to fight a War is… well, fighting. As you can probably imagine, Sieging an enemy’s Town is a great way to gain War Score. Participating in a siege regardless of winning or losing will grant you some war points. However, winning the siege (for Attackers that means capturing the Town Stash and for Defenders it means preventing the Attackers from taking the Town Stash) will grant you a small multiplier.
The other combat activity that will net you war points are outpost battles. Keep in mind that having active (undefeated) outposts will accrue you points in all active wars, so defending them is important.
It’s important to note that you may only be attacked during your vulnerability window. Make sure to set your nation’s vulnerability window at one of your town’s Battlemasters otherwise you will always be vulnerable.
The next two routes involve utilizing the help of the 3 major NPC Nations of Atlas, so we'll start with a quick explanation on how they factor in:
All 15 NPC Towns controlled by the 3 major factions (Purelakers, Vantians, and Shen-Cirilians) have a new NPC called the Quartermaster. Speaking to this NPC while your Nation is at War will allow you to take on War Quests in order to gain Diplomatic Favor with 1 of the 3 factions. Simply accruing Favor will contribute to your War Score - however utilizing that Favor with the NPC factions will net you even more points. But we’ll get to that…
The War Quests themselves are fairly simple, and don’t require forfeiting expensive materials to complete. There are a total of 120 War Quests, which are randomly given by the Quartermaster depending on which NPC Town you’re in. Some may require you to kill a local MythicMob, or deliver supplies to an ally, or bring materials to help build the town’s defenses - while others may ask you to go fight on the battlefield for that NPC Nation!
There are 3 types of Diplomatic Favor - Purelaker, Vantian, and Shen-Cirilian. Completing a War Quest will automatically award 1 Favor for the faction you’re questing for, and will bring up a GUI in which you can choose which War you want to contribute that Favor towards (if your Nation happens to be fighting multiple Wars simultaneously.)
So what is that Favor used for? I’m glad you asked! Because that brings us back to our second War path: the Political route. Accrue enough Favor with 1 of the 3 NPC factions, and you can spend that Favor to request that NPC Nation swear loyalty to you - meaning they will shut down that NPC Nation’s Towns to your enemies for the duration of the War! The NPCs in all 5 of that Nation’s NPC Towns will simply refuse to talk to your enemies - meaning no trade, no quests, and no fast travel. And of course, your enemies will no longer be able to complete War Quests for that faction for the duration of the War.
The third method of competing in a War is good ol’ fashioned money. Regardless of the Diplomatic Favor you’ve gained with an NPC faction, you can offer a bribe to the Quartermaster. If your bribe is accepted, that NPC Town’s merchants will double buy prices and halve sell prices for your enemies’ Nation members, for the duration of the war. Not to mention, each successful bribe will add to your Nation’s War Score.
Keep in mind, your enemies can bribe NPC Towns to make your prices worse, even in towns that you’ve already bribed. However, spending Favor to cause an NPC Nation to shun your enemies prevents both sides of the War from continuing to bribe that Nation’s Quartermasters. So plan your war strategy carefully!