So they went to Cersey for a truce to fight the knight walkers so Cersey master plan is to tell them her army is gonna fight with them in the north and then Not send the army
Options 1) Dont offer any shit and go with her plan
2) cheat them by sending an small army to earn their trust while she does all her shits
3) send a big army and attack them when they are busy fighting the deaths
4) offer an army, No sending anything so They notice straightaway Cersey betrays them by doing the exactly same shit they were asking her to do at first