War Game win conditions

Assassinate a certain target.
And if the game allows it, even being outnumbered could still win the day.

This works best with having to save up item cards. Until you have a chance in beating the target.

Until that time, you can duke it out in an old fashioned way.

Why does a war game have to have 1 victory condition?

There are many games...even war games, that have multiple winning conditions. It gives more depth in a game that otherwise would be one-dimensional (not that one-dimensional is bad).

See games like 7 Wonders: Duel, Legend of the 5 Rings Card Game, even classics like chess. It forces the player to plan resources carefully and gives a losing player a chance if one strategy is dominated by the opponent.

For this particular game description, I would consider ending the game once a player becomes strong enough to dominate over other players.

To determine that, you must first find a way to assess the "strength" of a player. An easy way would be to define "strength" as the number of regions controlled, but you can consider other metrics too (e.g., armies controlled, objectives, resources, or a combination of the above).

Then, determine what "strong enough" means. I would consider a condition like: "if the strength of a player is larger than the combined strength of any two other players". But you could also consider other conditions (e.g., achieving a predefined strength threshold). Another interesting alternative would be a condition that combines timing with strength, e.g., the sum of a player's strength and turns elapsed exceeds X; this has the additional advantage that it would prevent close games from dragging too long.

Finally, another idea you could consider (that I saw in action in a game I playtested recently) is to formalize the notion of alliances and define victory conditions for alliances. Since you have 6 factions, alliances will emerge anyway, so having explicit rules for alliances might be useful. In this case, you may have "winning player pairs".

Game of Thrones gives the victory once a player controls 7 castles. Points are a tried and true method (see the very successful memoir 44 and other command and colors games). Cry Havoc and Blood Rage both use points, and let even weak players continually recruit more units to hopefully get back into the fray and try to come out on top. (I especially like Cry Havoc's gems system that alters the value of each region over time)

I would look into some of the older and newer area control and warfare games since you are entering a new genre. Both for inspiration and to avoid reinventing the wheel :)

For your specific game, since there is a power vacuum, what if you have different factions whose loyalty you are attempting to earn? Each faction may respect doing well in a different part of the game, conquest, leadership, religion, etc. The game ends when a player has enough factions under their sway. (with 3 factions, maybe the first to earn loyalty from 2 of them?). Throwing in a time limit may also be a good idea (thematically, if all of the new powers are too weak by year 5, or whenever, the old power has had enough time to recooperate and comes back in to reclaim their territory)

I couldn't agree more. Except, please take THIS comment as an endorsement. It's probably the best game of the year.

In this way your game is very much like my game.

see http://thesingularitytrap.com/ to look over the game itself

but the win conditions in particular for mine are

Winning

Controlling 25 or more of the 37 planets at the beginning of 3 successive turns is a win for a single player.

Factions:

A winning faction may consist of 2 or 3 players in a 6 player game and 2 players in a 4 or 5 player game.

A faction is formed by declaration at the beginning of a turn where the faction holds 25 or more planets.

A faction must meet alliance requirements and hold 25 or more planets at the beginning of the following 2 turns to win.

A faction dissolves if required alliances are missing or the faction holds 24 or fewer planets at the beginning of a turn.

A 2 player faction consists of 2 players allied with each other and may be declared by either member.

A standard 3 player faction (with all members allied with all other members) may be declared by any member.

An empire faction is formed with a dominant player and 2 allies even if those allies are not allied with each other.
--- Only the dominant player may declare the empire and must control at least 13 planets by them self.
----- Empires additionally dissolve if a turn begins with 2 members not mutually allied and the dominant member below 13 planets.