The plot of the game makes absolutely no sense. At the end of KZ3, Helghan is utterly decimated. So where is the leverage coming from that would make the Vektans just give up half of their country? On top of that, why would any truce allow Helghast to remain MILITARIZED? And why would any third-party enforced truce not ensure safe passage over the wall?
It's a game so I really don't give a shit but it makes NO SENSE
Ok, so, if you remember in the original Killzone the Helghast had seized control of the highly advanced orbital platform orbiting Vekta and were going to use it to blow the ISA 'reinforcements from Earth' out of the Sky.
Those reinforcements are the UCN, 'Earth' and the ones that designate and arm ISA's throughout the human empire. The UNC is
dramatically more powerful and advanced than the ISA's or the Helghast, but travel still takes time adn communication isn't perfect due to the distance, that's why Vekta had to 'hold on' until UCN reinforcements could get there in KIllzone 1.
The ISA Vektan Cruisers used to invade Helghan? Dated hand me downs from the UCN.
But here's the problem, the UCN didn't authorize ISA Vekta to invade Helghan, they gave them those ships to contain and blockade Helghan while the UCN, not ISA Vekta, decided on further course of action. The Vektans wanted blood for the invasion of their planet and defied UCN orders.
This is probably why Vekta signs a 'non aggression pact' with the Helghast during KIllzone 3 an dordered all remaining ISA forces to surrender, the UCN was in the process dismantling the power structure of the ISA that lead to such a decision. Vekta had planned to swoop in, capture Visari and deal with the situation before the UCN knew or could do anything about it, and they'd get their revenge with the UCN being able to really punish them too much for disobeying orders.
Didn't work out that way. Invasion failed, tons of casualties, Visari dead not captured, or and billion die on Helghan. At that point the UCN probably wasn't in the mood to hear that it was an accidental chain reaction triggered by a rogue squad trying to stop an attack on Earth.
They needed to assert a stronger hand in the region.
- ISA, formerly 'Interplanetary Strategic Alliance - Vekta' is stripped of space fairing rights, now VSA, Vektan Strategic Alliance.
- Helghast survivors relocated to Vekta as Helghan is inhabitable. Large amount of sympathy/guilt from the Vektan populace of what happened and the UCN make this possible. However to prevent social collapse it's set up like east and west germany type of deal.
As for why the Helghast are militarizing, it could just be in contradiction to UCN decree. No idea, and apparently the VSA still have their ships as well, so huh.