[gensouR-1]

And to expand on it a bit:

Spoiler for expand:

1.due to the fact that NN and OU's territories are far away Nega Nebulous can't invade and declare war on Oscillatory Universe (you can't declare war on a Legion if your territory isn't near the opposing Legion's territory)
2.they need more allies to defeat OU because even though they have an alliance with Prominence,the combined force of NN+Prominence isn't strong enough to defeat White Cosmos,ARS,Seven Dwarfs and (1/5) Disaster MK2.
the purpose of the ceasefire is to create an alliance with Great Wall so NN+Prominence+GW can declare war on Oscillatory Universe/ARS with a chance of victory instead of no chance at all.