They have always been at war. There is no treaty with monsters.
Two non-human populations signed a treaty in 1983. It holds because silver still works on both.
Shepherd–Elder Accords
Contact pathology · DeterrenceCopperplate · OVERLORD · Plate IV · The Accords, 1983
They stopped fighting each other because they could not survive detection while doing so.
1983. A sealed bunker beneath a construction site on the most surveilled strip of land in human history. Representatives of two non-human populations signed a treaty no human government knew existed, in a language the Committee's linguists have not fully rendered. The Committee learned of it three years later. It did not recognise the intercept as significant until 2011. Twenty-five years. .
The 1968 lines were informal: haemophages the cities and the corridors, Lycians the dark hinterland, enforced by avoidance. The Accords formalised convergent desperation. Both faced the same squeeze. Neither could survive it while fighting the other. The enforcement is biological: silver works on both. Mutually assured destruction without a warhead. It has held because neither side has made it asymmetric.