Lord Krishna in his sermon of Bhagavad Gita says when life comes full circle, when society reaches its lowest ebb advents a messiah an avatar, a savior to salvage mankind from whatever is left. Lord Krishna advent in Dwapar Yuga when strife, tribulations were at its peak!
In present Kali Yuga Lord Krishna says he would advent as Kalki Avatar, annihilator of Kali Yuga to cleanse the society of its ills. The advent of an avatar is solely for one purpose re-establishing dharma (righteousness) against evil forces of adharma (lawlessness).
During times of Lord Krishna mankind witnessed mother of all wars battle of Mahabharata. In the present when conditions are not conducive for society to survive any further the advent of Kalki Avatar would announce beginning of world war 3, a full scale nuclear war, man-made calamity.
As per Lord Krishna in Bhagavad Gita life comes full circle every 3500 to 5000 years. This means advent of an avatar every 3500 to 5000 years is mandated by god. None could change this destiny of mankind. In a Kali Yuga almost 95% people were impure, indulged in negativity. On the contrary in a Satyuga about 95% people were truthful, indulging in positivity most of time.
Only a few years from now mankind would witness advent of Kalki Avatar, the fiercest man animal to advent on mother earth. A one-man army having a dedicated following of above 2000 million people worldwide. In no time he would succeed re-establishing Dharma (righteousness) and start Satyuga (the awaited golden period).