It couldn't have taken place on Harvest because there was no battle at Harvest. The Covenant glassed it on the spot. If there was a ground fight, then they would've radioed for help.

 It couldn't be Mombasa or anywhere on Earth because if it takes place before Halo CE, the Covenant hasn't found Earth. They found Earth by Halo 2.

 It's not likely to be on Reach because there wasn't any description of an artic battle at least not in Fall of Reach.

Can't be on a Halo because Halo wasn't discovered until years later, if it takes place before Halo CE. And if so, then Spartan 3s didn't come into play until much much later in Ghosts of Onyx. The humans had nothing when the Autumn crashed on Halo. They were all disbanded, separated, with few weapons and supplies and only ONE Spartan(MC). Pelicans were limited. There was no description that battle rifles and artic camoflague was used. No human fighters, whatever type they are except a few Longswords. And I doubt they can coordinate an counteroffensive like that.

Best guess: Unnamed planet as of yet.

Here's what think;

The Covenant were first encountered in 2525 at Harvest while Reach fell in 2552. That gives us a 27 year time frame. Because it takes place at the beginning of the Human-Covenant War, we can narrow down the time frame and possible location.  I believe that it takes place somewhere in the Outer Colonies. That, or it obviously takes place somewhere else. Theres my idea.

If we stop guessing and start stating facts we could narrow down the possibilities.

 What I can definately say is that this is the anywhere from the first to third contact with the covanent. There could of been a ship on patrol that found something, checked it out, and was destroyed before word was sent back of the true first contact. What makes it have to be so early on in the war is the quote from the spartan at the end of the trailer, "If they want war, we'll give them war." Thats something you don't say half way through a war, its something you say at the VERY beginning of one.
 

Correct, but since we are reading into things..... Just because they use the word 'war' doesn't mean there weren't battles before this instance.  There may have been skirmishes or encounters that were violent.  He is a SPARTAN, not a general, he is not qualified to use the word 'war' as a declaration of one.  He could have merely been saying "if they want a fight, we'll give 'em one." 

 On top of all this, it was a promo video....made to give us the emotion of the game...it may not be an accurate representation of the true timeline.

TstclrCncr:

If we stop guessing and start stating facts we could narrow down the possibilities. 

So true.

wyattFNearpp:

Just because they use the word 'war' doesn't mean there weren't battles before this instance.  There may have been skirmishes or encounters that were violent. 

 Sorry about this contradiction wyattFNearpp: this Halo Wars game (hopefully everybody now knows) is said to be set before the game Halo: Combat Evolved. The reasons for the Spartans could well be that they part of the SPARTAN-1 project. MC and the rest were part of the SPARTAN-2 project. Also the reason for this is that when the the leader of these Spartans says "if they want a war we'll give 'em one", this I think shows that it is definitely nearer the beginning of the Human-Covenant encounters. In the first book Halo: The Fall of Reach, i think the first encounter of the Covenant is when the Spartan-2s are being trained in some underground cavern on Reach?

However, when some of the Spartan-2s fought against the Covenant on Sigma Octanus 4, in Cote d'Azur, there were only 12 of them. There were still 20 other Spartan-2s not there, so they could have been transferred to fight the Covenant on other planets which have been un-named. These facts and theories may help provide you with enough information to think of others, or whatever. I just hope it helped narrow down the possibilities.

:p Tuek

Sorry about this contradiction wyattFNearpp: this Halo Wars game (hopefully everybody now knows) is said to be set before the game Halo: Combat Evolved. The reasons for the Spartans could well be that they part of the SPARTAN-1 project. MC and the rest were part of the SPARTAN-2 project. Also the reason for this is that when the the leader of these Spartans says "if they want a war we'll give 'em one", this I think shows that it is definitely nearer the beginning of the Human-Covenant encounters. In the first book Halo: The Fall of Reach, i think the first encounter of the Covenant is when the Spartan-2s are being trained in some underground cavern on Reach?

However, when some of the Spartan-2s fought against the Covenant on Sigma Octanus 4, in Cote d'Azur, there were only 12 of them. There were still 20 other Spartan-2s not there, so they could have been transferred to fight the Covenant on other planets which have been un-named. These facts and theories may help provide you with enough information to think of others, or whatever. I just hope it helped narrow down the possibilities.

:p Tuek

 Finally someone who also agrees that it happens near the beginning! I want your babies!
 

TstclrCncr:

  Finally someone who also agrees that it happens near the beginning! I want your babies!
 

 Steady mate! I'm not sure if this happens near the beginning, i just gave some ideas, or reasons why it could be.