They were with Kael’thas, just he himself part of the army and ofc not there as a contigent of elves alone (was not the point).
An human emissary communicates Garithos’ orders, to the Prince,that they are to stay at the tower/observatory (I can’t remember) they repaired and stop a laughably big amount of Scourge before they reach Dalaran with his own men (Garithos orders were to dismiss everyone else) . After the elves survive thanks to the help of nagas Garithos himself arrives and has the whole force litterally imprisoned.
Im not saying the alliance as whole but at least the guardians of said prison weren’t really “polite” at the elves imprisoned there, which strongly hints to racial hatred of a minor/major group.
The order was to execute them, so they escaped.
Now you tell the survivors of that realm that their Leader was to be executed by an officer of their former allies.
The incident itself is legit big enough and not as lame as some people depicts it.