According to Srimad-Bhagavatam the appearance of Lord Balarama is stated as follows:
there, when the earth was overburdened by demonic rulers who had amassed large military forces armed with deadly weapons to wage war for personal gain, Lord Brahma, the chief of the demigods in this universe, approached the shore of the ocean of milk. There, accompanied by Lord Siva and other celestial beings, he worshiped Lord Visnu, the supreme God of all gods. In response, Lord Visnu imparted some instructions, which Lord Brahma in turn conveyed to the demigods. He told them that very soon the Lord would appear on earth to diminish the burden of the world and that the demigods, to assist Him, should take birth in the Yadu dynasty. Lord Brahma further stated that Lord Balarama, who is also known as Sankarsana, would precede the Lord and serve Him in every respect:
“The foremost manifestation of Krsna is Sankarsana, who is known as Ananta. He is the origin of all incarnations within this material world. Previous to the appearance of Lord Krsna, this original Sankarsana will appear as Baladeva, just to please the Supreme Lord Krsna in His transcendental pastimes.” (SB 10.1.24)
Lord Brahma told them as well that Yogamaya, the personal potency of the Lord, would also appear and assist the Lord in His mission.
Many of you may know the history that immediately preceded the appearance of Krsna and Balarama. Vasudeva, a great devotee in the Yadu dynasty, wedded Devaki, another pure devotee. After their marriage, Vasudeva mounted his chariot with his newly wedded wife to return home, and Devaki’s brother Kamsa, to please his sister, took the reins of the horses to drive the chariot. Along the way, Kamsa heard a voice from an invisible source, which told him, “You fool, the eighth child of the woman you are carrying will kill you!” Kamsa, a great demon, became enraged and was ready to kill his own sister on the day of her marriage. But Vasudeva intervened and appealed to him to desist. He said, “How could a qualified person like you kill a woman–your own sister–on the occasion of her marriage? In any case, one day you will die, and in your next life you will have to suffer the reactions to your present activities.” Still, Kamsa was so shameless and cruel that he persisted, and Vasudeva, to save his wife from imminent death, told him, “You have nothing to fear from Devaki, according to the omen. It is her son that will kill you. So I promise to deliver all her sons, who are the cause of your fear, into your hands.” Kamsa, having faith in Vasudeva’s word, was pacified. Still, he imprisoned Vasudeva and Devaki. And year after year, as their children were born, Vasudeva very dutifully delivered them to Kamsa, who mercilessly killed them.
The Lord ordered Yogamaya: O My potency, who are worshipable for the entire world and whose nature is to bestow good fortune upon all living entities, go to Vraja, where there live many cowherd men and their wives. In that very beautiful land, where many cows reside, Rohini, the wife of Vasudeva, is living at the home of Nanda Maharaja. Other wives of Vasudeva are also living there incognito because of fear of Kamsa. Please go there.
Within the womb of Devaki is My partial plenary expansion known as Sankarsana or Sesa. Without difficulty, transfer Him into the womb of Rohini.
O all-auspicious Yogamaya, I shall then appear with My full six opulences as the son of Devaki, and you will appear as the daughter of Mother Yasoda, the queen of Maharaja Nanda.
When Nanda Maharaja learned that the Yadus had arrived, led by Krsna, he immediately went to see them. The cowherds accompanied him, their various possessions loaded on their wagons.
Seeing Nanda, the Vrsnis were delighted and stood up like dead bodies coming back to life. Having felt much distress at not seeing him for so long, they held him in a tight embrace.
Vasudeva embraced Nanda Maharaja with great joy. Beside himself with ecstatic love, Vasudeva remembered the troubles Kamsa had caused him, forcing him to leave his sons in Gokula for Their safety.
O hero of the Kurus, Krsna and Balarama embraced Their foster parents and bowed down to them, but Their throats were so choked up with tears of love that the two Lords could say nothing.
Raising their two sons onto their laps and holding Them in their arms, Nanda and saintly Mother Yasoda forgot their sorrows.
Then Rohini and Devaki both embraced the Queen of Vraja, remembering the faithful friendship she had shown them. Their throats choking with tears, they addressed her as follows.
[Rohini and Devaki said:] What woman could forget the unceasing friendship you and Nanda have shown us, dear Queen of Vraja? There is no way to repay you in this world, even with the wealth of Indra.
Before these two boys had ever seen Their real parents, you acted as Their parents and gave Them all affectionate care, training, nourishment, and protection. They were never afraid, good lady, because you protected Them just as eyelids protect the eyes. Indeed, saintly persons like you never discriminate between outsiders and their own kin.
“Gargamuni said: This child, the son of Rohini, will give all happiness to His relatives and friends by His transcendental qualities. Therefore He will be known as Rama. And because He will manifest extraordinary bodily strength, He will also be known as Bala. Moreover, because He unites two families–Vasudeva’s family and the family of Nanda Maharaja–He will be known as Sankarsana.”(SB 10.8.12)
Gargamuni did disclose to Nanda Maharaja that Balarama would be known as Sankarsana because of uniting two families–the yadu-vamsa and the vamsa of Nanda Maharaja–one of which was known as ksatriya and the other as vaisya. Both families had the same original forefather, the only difference being that Nanda Maharaja was born of a vaisya wife whereas Vasudeva was born of a ksatriya wife. Later, Nanda Maharaja married a vaisya wife, and Vasudeva married a ksatriya wife. So although the families of Nanda Maharaja and Vasudeva both came from the same father, they were divided as ksatriya and vaisya. Now Baladeva united them, and therefore He was known as Sankarsana.”
By Krsna consciousness, by cooperating to serve the spiritual master and the Supreme Personality of Godhead, different individuals, families, castes, and communities can all be united.
Even Balarama considers Himself to be a servant of Lord Krsna. Balarama is the first expansion of Krsna and is the original spiritual master, and He too has the sentiment that He is a servant of Krsna. In fact, every conscious being is imbued with the sentiment of being a servant of Krsna. Balarama, and all of Krsna’s expansions–Vasudeva, Sankarsana, Pradyumna, Aniruddha, Karanodakasayi Visnu, Garbhodakasayi Visnu, Ksirodakasayi Visnu–all have the sentiment of being servants of Krsna.
ekale isvara krsna, ara saba bhrtya
yare yaiche nacaya, se taiche kare nrtya
“Lord Krsna alone is the supreme controller, and all others are His servants. They dance as He makes them do so.” (Cc Adi 5.142)
apanake bhrtya kari’ krsne prabhu jane
krsnera kalara kala apanake mane
“[Balarama] considers Himself a servant and knows Krsna to be His master. Thus He regards Himself as a fragment of His plenary portion.” (Cc Adi 5.137)
It was stated in the beginning that Balarama, or Sankarsana, would appear prior to Krsna in order to serve Krsna and please Him in every respect.
“The foremost manifestation of Krsna is Sankarsana, who is known as Ananta. He is the origin of all incarnations within this material world. Previous to the appearance of Lord Krsna, this original Sankarsana will appear as Baladeva, just to please the Supreme Lord Krsna in His transcendental pastimes.” (SB 10.1.24)