And why would UEF know to attack these ships from behind?
Are you serious?
From the destruction of whatever ship they send as first reaction (like the Renjian) they'd know that the front is bristling with weapons. If you can't get them from the front, try the other side, especially when that other side houses such massive engines that there is almost no room to put turrets on.
I don't think they'd take such precautions on their first attack, or even be able to jump so precisely.
They did with the two Narayana that attacked the Atreus in Darkest Hour.
Also, their armament and fighters would be much weaker than at the time of WiH.
The Renjian was fully armed, the same way the ships in the campaign where, right when the first shot of the war was made.
Not to mention nobody would risk sending frigates when GTVA could launch a volley of missiles at key cities on Mars and Earth the second a subspace vortex started to open.
If we take the way ships act ingame as canon for what they can and can't do, they can't launch missiles from that far away. And even if they did, those torpedoes are so damn slow they can easily be shot down in time before they reach the atmosphere.
It is utterly ridiculous that one destroyer, two corvettes and two cruisers might lose, when attacked by five Karunas from behind, which are supported by strikecraft vastly superiour to their own?
they would have to know to attack from range to make it work as the GTVA warships would easily manoeuvre to bring their beams in full view.
Actually they don't. I just tried a little experiment in FRED with a Raynor, Bellerophon, Chimera and Hyperion against five Karunas from behind. The GTVA had three fighter and two bomber wings (all with 3 waves and 4 ships each), the UEF had one Uriel wing (4 fighters 1 wave.... actually only 3 combatants, since I was sitting back watching), one Uhlan, one Kentauroi wing (with 4 ships and 2 waves) and a wing of three Durga (with 2 waves).
The GTVA strike craft where armed with Balor, Prom S (Kayser for wing leader) and Maxim according to their role as primaries and Harpoon/Tornado for fighters, Tornato/Cyclops/Helios for Boanerges, Helios/Stiletto for Rhea (orderd to disarm Karunas) as secondaries.
UEF fighters were default equiped for the most part, except the Uriels, who'm I gave Javelin/Hellfire/Jackhammer (in anticipation of me ordering them around to fill what role needed filling... which I neglected in favour of watching). All ships had BP-Captain as AI profile and the difficulty was set to medium.
So even with the GTVA having top equipment on their fighters and having more fighters, the GTVA lost three out of three times. Twice two Karunas survived, once it was three survivors.
The battles were over before they degenerated into a circling match, but the mass drivers, gauss cannons and torpedoes made such short work of the frontal beams, that the GTVA only got off two salvoes of beamfire.
I know this doesn't proof that the UEF always wins such a fight, but it shows that it is at least a real possibility, or maybe even likely, even though they were well within beam range right from the start.