Kaikeyi's Mind Poisoned By Manthara
Manthara ran to Kaikeyi's palace and broke the news. Kaikeyi's first reaction was to bestow some gifts on Manthara for the glad tidings she had brought (Valmiki R/2/7/32). But throught clever and persistent arguments, Manthara poisoned Kaikeyi's mind. "Bharata's departure to Kekaya was a part of Dasaratha's conspiracy to deprive him of his inheritance,"she said. She added that Kaikeyi would have to wait upon Kausalya, as her maid, if Sri Rama ascended the throne. That would spell danger to Bharata too.
Manthara found that Kaikeyi was listening. She elaborated on her scheme. Sri Rama could be sent to the forest, and Bharata could be installed King. Sri Rama's stay in the forest could be for fourteen years to give sufficent time for Bharata to consolidate his position.
The story goes that once King Dasaratha had helped the Devas in their fight with the Asuras, especially Sambarasura. In that war, Kaikeyi as a charioteer had saved her husband Dasaratha when he fell down unconscious. The grateful King promised her two boons (Valmiki R/2/9/17). Kaikeyi had kept the promise in reserve to be utilized at an appropriate time.
Manthara now advised Kaikeyi to encash those two boons by which Bharata should be enthroned as the king, and Sri Rama be sent to the Dandaka forest for a period of fourteen years.
Accordingly, Kaikeyi was to feign anger and disappointment and extract a promise from Dasaratha, a promise that he would act on his earlier promise to grant boons. Only then Kaikeyi would reveal her plans. Kaikeyi and the maid agreed on the conspiracy.