Previously, the demigods and demons had been at war in the heavens. When the demons were defeated by the demigods, they decided to instead attack this planet earth. Thus, they invaded the earth by discretely taking birth as princes in powerful royal families of the time.
And as the earth became overrun by militaristic activities of these kingly demons, the demigods including the Earth goddess earnestly sought Lord Visnu's protection. Seeing the deteriorating social and political conditions and hearing the prayers of the demigods, the all-compassionate Supreme Lord Sri Krishna decided to descend for the benefit of all.
The Supreme Lord descends from time to time in this material world to reestablish the teachings of the Vedas. In His Bhagavad-gita, Lord Krishna promises: "Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion--at that time I descend Myself. To deliver the pious and to annihilate the miscreants, as well as to reestablish the principles of religion, I Myself appear milleniumm after millenium."
Although eternal the Lord appears in specific circumstances out of mercy for His devotees. In fact, His principal biography, the Srimad Bhagavatam states, "the learned men describe the births and activities of the Unborn and Inactive." Therefore, although He appears within the material dimensions of time and space, He is most definitely not of it.
Historically, Lord Krishna appeared on the midnight of the 8th day of the dark half of the month of Sravana. This corresponds to July 19th 3228 BC. He exhibited His pastimes for a little over 125 years and dissappeared on February 18th 3102 BC on the new moon night of Phalguna. (His departure marks the beginning of the current age of corruption known as Kali.)
The great scholar Srila Vishvanatha Chakravarti neatly outlines Lord Krishna's activities in this way: the first three years and four months were spent in Gokula, then equal lengths of time in Vmdavana and Nandagram, eighteen years and four months in Mathura, and finally ninety-six years and eight months in Dwaraka totalling 125 years of manifest pastimes. See the Krishna-lila chart.
Lord Krishna's early pastimes are briefly summarized at the website Krishna's Adventures in Vraja"During this childhood time , He grew up as the son of His foster parents Nanda and Yasoda in the midst of the idyllic beauty of Gokula, Vrindavana, and Nandagram. Not only did He destroy numerous demons, but also performed His famous rasa dance.
Krishna enjoyed the dance of love (rasa-lila) with the gopis many of whom are expansions of His own internal energies. The supreme gopi known as Srimati Radharani is the object of Krishna's highest devotion. This beautiful dance would occur in the autumn season at night under a full moon when Lord Krishna would captivate the young gopis with the extraordinary music of His flute . These esoteric pastimes constitute the most confidential expression of divinity ever revealed.
Usually the conception of sprirtual perfection consists of overwhelming feelings only of awe and reverence at God's majesty. However, in these pastimes each devotee loves God either as a master, a best friend, a mischievous son, or even as an intimate lover, thus revealing the infinite possibilities of divine love. These early pastimes of Lord Krishna in Vrindavana illustrate the extraordinary intimacy that one can have with God. These pastimes are described in detail by Sri Visvantha Chakravarti in his Sri Krishna Bhavanamrta Mahakavya
When Krsna and Balarama were older, They were invited to Mathura, where Karnsa, Their demonic uncle, was planning Their death in a wrestling match against two large and powerful wrestlers. When Kamsa saw his wrestlers defeated, he ordered his friends to drive the brothers out of Mathura, plunder the riches of Their cowherd friends, and kill Their fathers, Nanda and Vasudeva. However, Krishna immediately killed Kamsa and Balarama killed his eight brothers. Lord Krishna then established the pious King Ugrasena as the emperor of several kingdoms.
In Mathura, both Krishna and Balarama were initiated by Gargamuni in the Gayatri mantra> Later They went to live under the care of Sandipani Muni who instructed Them in all the Vedic arts and sciences in sixty-four days and nights especially in military science, politics and spirituality. As an offering (guru-daksina) to Their teacher, They recovered his son from death. Although God does not need instruction from anyone else, Lord Krishna and His brother set the perfect example : one must accept instruction from and serve a bona-fide spiritual master to advance in spiritual life.
For the next eighteen years, They continued to live in Mathura halting the impending threat of many demonic kings. Later in Their pastimes Lord Balarama married a princess named Revati. Lord Krsna married many queens, the foremost among them being the extraordinarily beautiftil Queen Rukmini. (See Sri Rukmini website for the story of Their marriage). Both Krishna and Balarama established Their palaces in Dwaraka off the coast of western India, where They enjoyed married life for many years. Although They were married, Lord Krishna and Lord Balarama exhibited the quality of detachment from material life perfectly.
When They were about ninety years old, the great world war of Kuruksetra took place. This climactic battle brought together all the major world leaders. Lord Krishna took the role of a charioteer on the side of the pious Pandavas, while Lord Balarama refusing to participate went on a pilgrimage tour thereby blessing the entire land of India.
At the start of the war, Lord Krishna displayed His stupendous Universal Form delivered His famous message known as the Bhagavad-gita, literally the Song of God. This Song contains the essence of all knowledge having been spoken by the Supreme Lord Sri Krishna Himself. This war concluded with the destruction of the demonic kings and the reinstatement of the righteous Pandava princes.
Having completed Their mission, Balarama and Krishna resumed Their life in Dwaraka where They spent some thirty-five more years before ending Their earthly manifest activities The foremost description of Lord Krishna's activities occurs in the Srimad Bhagavatam, literally "the Beautiful Book of God."