Any match (unless private) played in For Honor contributes to the global cross-platform Faction War.
Three factions: Knights, Vikings and Samurai confront each other to control more territory.
Here is a map of Faction War:
The zones here are named and colored according to the initial situation at the beginning of a season. Obviously, the territory Faction control changes in the course of War.

Each Season (10 weeks) consist of 5 Rounds (2 weeks), every Round has a winner according to the territory Faction controls at the end of it.
Each Round consist of Turns (6 hours). At the end of each Turn Factions gain and lose their territory according to the player actions.
Each player chooses one of the three available Factions before he starts playing. He can change his Faction after that, but then he will not get rewards for the current Season. You get rewards at the end of each Round depending on your Faction success. And some bigger reward at the end of Season.
The current situation and the history of Faction War can be viewed at the official site of the game[game-forhonor.ubisoft.com].

Faction War map viewed in game or at the website shows not only which Faction controls the territory, but the battle advantages at this Turn.
You can see relative War Assets advantage plotted over each disputed territory. The colored swords icon shows which Faction will gain the territory if the situation will remain unchanged.

The territory is conquered (protected) by Faction which has more War Assets invested in it. Be it 51% of War Assets or 90% (or even 34%, if two other Factions have 33%), the territory will belong to the Faction which has the biggest part of War Assets. At the next Turn all War Assets should be invested again, from scratch. That means that you would not want to overinvest too much of War Assets in one territory, while losing another by 1%.
The game has system of averaging overall War Assets of Factions to make the Faction War balanced, so the thing is more about distribution of the available resources. Overpopulated Faction will not have much more War Assets than Faction with less players.

You can invest your War Assets after each game protecting the territory of your Faction or conquering the territory of other Factions. If you will not invest gained War Assets yourself, the game will split them even over all territories for you.
You can select where do you want to deploy your War Assets after each multiplayer match (in the NEXT MATCH menu - DEPLOY WAR ASSETS option). To deploy them to some territory not corresponding to the game type played press Esc.
The War Assets you gain depend on your performance during game and various other reasons you can not influence.
On the map there are three conflict zones, each of them corresponds to some game mode. You can invest your War Assets in every disputed territory, but you invest 20% more War Assets in the territory of the corresponding game mode you have played.
Additionally you invest 20% more War Assets when the end of Turn is near.

All the territories Faction controls should be connected to that Faction core zone (K0, V0 or S0), or else they will be assimilated by the Factions surrounding them at the next Turn.


So the enclave territory can exist only for one Turn, being connected to the core zone by Faction territories or being assimilated by other Factions next.


That also means that you do not want to conquer territories in the middle of nowhere, as they will be surely lost, while War Assets invested to conquer them could've served greater good.


Sometimes it can happen that territories cut each other off.


Then they will be redistributed the next Turn according to the number of territories not surrounded.

For now there is almost no coordination for player actions. No option provided by the game itself, no major community associations.
Some players just do not distribute their War Assets (it is their right), they are distributed evenly. Some other players are probably just distributing them randomly, without general strategy. This provides quite chaotic picture.

It is hard for one Fraction to stand against combined efforts of two other Fractions. But when one front is pressed, the other remains undefended.
You can see here that, even being cornered, Knights could not take back even one territory. They tried to push back Samurai who invested most of their War Assets to this frontline. They were close, but could not do it. While Samurai and Knights were occupied with each other, Vikings advanced on all directions almost freely. Should've they pressed Knights further, who knows...
But Samurai moved to protect their other border, and Knights managed to take back some of their lands.

It may be possible to add some depth to Faction War, but for now it is too chaotic to talk about strategy. To some extent, it may be caused by poor understanding of how things do work.
It is not like I am expert on the field myself. I just tried to describe the things I do understand by playing the game. If you have something to add or to correct, please do.

Best regards.