Surrender

Let me try to be more specific. Perhaps that can elicit a respons 

I was at war with the Torians, grinding them down surely but slowly. I was also allied with  the Thalans and Altarians, so I would get a diplomatic victory by eliminatint the Torians.

Tha Altarians were allied with the Torians (and me) and was in the process of eliminating the Thalans.

I looked repeatedly for an option in the Diplomacy screen to get the Torians to surrender to me. It didn't appear. I wonder what it takes to get that option - it is not very apparent. Eventually, they surrendered to the Altarians, and I won the game a turn or two later. Before that, the Thalans also decided to surrender to the Altarians. I was an ally with the Thalans, so they could have decided to surender to me. I wonder why they decided to surrender to their enemy, contrary to the Torians, and not to their ally, as the Torians did.

As I said, I don't understand this diplomatic model, and I don't understand what to do to get the effect I want. I'll be happy to gain some insights from people who are more experienced with the game than me.

You can't request that someone surrender-to you, or anyone else, for that matter.

What you can do is bully them into giving you all of their stuff.  Generally, this takes having a military >5x theirs, and may even require a military >10x theirs.  This is a weak point in the AI in that it overvalues military might and undervalues...well, everything else...but there doesn't seem to be anything to do about it at present except use it.

Typically this is done by declaring war on them or having them declare war on you with your monstrous military (park transports in their systems and they'll become very angry at you) and then when they ask for a peace treaty taking everything from them for it.  In DL it could be done on the same turn, and while it is still possible in DA/TA to do so, I won't ruin that for you.

For what it's worth, perceived issues with the surrender algorithm have been a topic as long as I can remember.  Other than not having a military, to my knowledge no one has yet figured out exactly why, when, or who the AI will choose to surrender to.

Thanks, that is helpful. I wouldn't risk peace with them as they were allied with the Altarians, so declaring war on them again might give me a war with the Altarians and thus ruin my diplomatic victory. I did try to see how they reacted to giving me a planet, but it was always a no-go (red text). So I gather the method in a similar situation is to simply keep going until they will give me all their planets, or they surrender to someone.

Pretty much-although it's moreso "have a massively larger military" than "keep going".  (Both work-the former just works -better-.)

With the exception of the Altarian super ability, which interferes in very strange ways, if you are allied with race A who is allied with race B and you declare war on race B, race A does not get a prompt to go to war with you, or race B for that matter-and therefore will not.

The Altarian super ability alters things slightly-any time someone declares war on someone with super organizer, all same-alignment or same-leaning players (including AIs) get a prompt (which, unlike that which pops up if it's alliance-based, has no consequences if you say no), which the AI will always agree to assist, provided that their relations with the super organizer race who is being attacked are neutral or better.

In other words, don't worry about attacking them-your mutual allies will not interfere on either side.

Thanks again. That's really nice to know.

The manual  isn't all that specific and I havent really found another place but this forum to get info like this. Is therea FAQ or similar with relevant info?

Try the wiki.  The Library link on the left has a link to it IIRC, but it's broken.

For what it's worth, to my knowledge the information I provided above does not (yet) exist on the wiki.