Thinking back on it now, it makes little sense; Adora was the Princess of Eternia, the kingdom if not the planet itself. And in various flashbacks, the Sorceress says that the Horde tried to invade Eternia as they had countless other planets, but "they were no match for your father's brave soldiers, and the magic of Castle Greyskull". You can see it here:
Hordack himself even says "You may have defeated us, but you'll never see this child again!" when he escapes to Etheria, and later in the Secret of the Sword film says "The Horde has no more interest in this world!" [likely because they got their asses handed to them!]
Considering the Horde is purported to be a galaxy-spanning empire, Eternia must be one TOUGH ass planet of Badasses to have them just straight "Nope!" off the planet and never return!
But then.... why didn't the Eternian Army come to aid their Princess in a rebellion she was a part of for years?
I mean obvious separation of the series aside.... wouldn't that have ended the war sooner?
Think about it:
- The Eternian Army---- not even the PLANET-wide army, just (it seems) those under King Randor---- were more than a match for the Horde. So they had an established success against the evil forces already.
- Even if they wouldn't have Grayskull's magic on their side this time, they would still have the Crystal castle, or at least the Kingdoms of Bright Moon and its allies like Spike-Heart. So magically, they should be covered.
- They had adequate reason to join the fight, as the Horde had kept their Princess as a POW for at least 18 years! Even if they want to claim that a whole other planet is somewhat "out of their jurisdiction," as a foreign power, they can offer aid to a friendly kingdom under siege from a common enemy. Adora's imprisonment, as a War Crime, would be justification enough for them sending troops or supplies as aid to the Etherian realms as a means of support. Not only would they be protecting their interest (Adora) they could argue being "pulled" into the conflict due to "previous aggressive actions against the kingdom of Eternia" by the Horde.
And its not as if they were engaged in any other serious conflict; Skeletor was a continuous threat, yes,but not one the army is called in to do with. His villainous band is more of a "Terrorist cell" trying to make incursions into the kingdom, not on the same scale as full other nation waging endless battle. Sending troops to aid the Rebellion as a means of assuring Adora's safety seems logical.
And yeah, the Rebellion was going on for years, even if we presume it only started a few months back before or right at the beginning of Secret of the Sword. This to makes sense, as by that time Angella had been captured, Adora had been raised on the planet as a trained and established Force Captain, and a good portion of Etheria had been under Horde rule and taxation for years. This had been made worse by an apparent "time skip" that had occurred between seasons 1 and 2, a time skip made clear by the life of General Sundar.
The Rebellion ran into Sundar in season 1, and they'd been fighting alongside She-Ra for some time by then. Adora had been with the Rebellion as one of their co-leaders after Secret in season 1, before Sundar had switched sides; when Sundar returned in season 2, Adora was almost Completely in charge of the Rebellion by then, and Sundar and Mally had a daughter who was at least 3 or 4 years old. If the war had been continuing through All that time, then that's most likely 5 years of battle with the Horde!
Granted, time probably moves differently on Eternia and Etheria, but it's still a long period for a Princess to be at risk and a king of a relatively peaceful nation to not offer any support.
I know there was a post-2000s comic which may have gone into details [have no clue, honestly], but is there any canon reason why no suggestion of aid was given?