March Gītā: 4.7

yadaa yadaa hi dharmasya glaanirbhavati bhaarata
abhyutthaanamadharmasya tadaatmaanam srijaamyaham

‘Whenever there is a decline in righteousness, and a rise in unrighteousness prevails, then do I manifest myself, O Bhaarata.’
Bhagavad Gītā 4.7

Dharma is a word with many meanings and, in the context of this verse, it can be translated as harmony so that adharma becomes discord or dis-harmony. 

You could read this verse and think that Krishna is like a Messiah: waiting to return to a broken world. But in a previous sloka he has described himself as an avatar. Krishna is a representation of qualities which dwell in the heart of all beings. So he is there in every act of love, kindness, compassion or bravery. The qualities of Krishna are also embodied in the text of the Gita, a text to which we ourselves can return in times of trouble. 

– Tom Norrington-Davies