Examine the end of Act I scene 1 very carefully, when Goneril and Regan discuss Lear's decision to spend time with each of them for a month before moving on to the next daughter. They have just witnessed their sister, who they both knew to be their father's favourite, banished, and Kent, one of their father's most loyal supporters, banished as well. They clearly are beginning to suspect that their father is at best irrational and at worst affected by his advanced age. Note what Goneril says to Regan at the end of this scene:
There is further compliment of leave-taking between France and [Kent].
Goneril is clearly suspicious about Kent's motives for speaking with the King of France, and this is something that becomes more acute as the play progresses and Goneril and Regan start to use and abuse their father's power that he has so unwisely bestowed upon them. Regan and Goneril do nothing to try and protect Kent or Cordelia from their father's capricious anger, and so they can be said to be part of the reason for the war that develops between France and England.