(So when there is satanic influence over the kingdom of God, or the devotees, or the appointed demigods, they are disturbed by satanic influence, then Kṛṣṇa comes) 
"So when there is satanic influence over the kingdom of God, or the devotees, or the appointed demigods, they are disturbed by satanic influence, then Kṛṣṇa comes. Yadā yadā hi . . . that is stated, confirmed, in Bhagavad-gītā.
yadā yadā hi dharmasya
glānir bhavati bhārata
abhyutthānam adharmasya
tadātmānaṁ sṛjāmy aham
(BG 4.7)
Whenever there is discrepancies in the . . . in the discharge of natural laws of God, then God incarnates. Either He incarnates Himself, or He sends His bona fide representative, or He sends His powerful representative, like that. So whenever there is incarnation of God, it is to be understood that there is discrepancies in the matter of discharging the laws made by God."
Comments
So often the prophets are attacked by the demonic forces. There is no peace down here for long.
Yes prabhu, the fight between the Demons and Demigods is taking place perpetually in this material world... and the demons are running amok in this age of Kali
Devotee: Why was, why was Lord Rāma letting Rāvaṇa live in Laṅkā?
Prabhupāda: Eh?
Devotee: Why was He permitting him to live there? He was irreligious. He did not trust in God. Why was Lord Rāma letting him . . .?
Prabhupāda: No, no. He was killed therefore. He was not allowed to live there. Therefore Rāmacandra went there to kill him. "You rascal. You must be killed." Why do you say that he was allowed?
Devotee: Well . . .
Prabhupāda: He was not allowed.
Devotee: For some time. For some time.
Prabhupāda: For sometimes every thief flourishes. That is another thing. But he must be punished.
Devotee: Hmm. He was punished.
Prabhupāda: You can cheat for some time everyone, but you cannot be allowed to cheat everyone for all the time. That is not possible. (break) . . . law. Therefore Lord Rāmacandra could create millions of Sītā. So the purpose was to kill this atheist.