Bhagavad gita is a part of Mahabharata and therefore the background of the Mahabharata story can help us to better understand these first verses of Gita.

King Santanu fell in love with Goddess Ganga and married her, begetting an extraordinary son called Devavrata (later called  Bhishma). However, Santanu’s limited intelligence was unable to understand or even just to accept the higher level of motivations and plans of the Devas, therefore when Ganga asked him to respect one single condition for their marriage, Santanu accepted without expecting that such condition would be difficult to bear. The condition was that  Santanu could never question Ganga’s actions. So when at the birth of their first child, Ganga went to the river to drown the baby, Santanu was deeply disturbed. He kept his silence while Ganga drowned the first  seven babies and but was unable to accept any more when Ganga took their eighth baby to the river too, and he tried to stop his wife. Having violated the agreement, Santanu was left by Ganga who took Devavrata to the higher planets for his education and sent him back to his father when he was ready to rule the kingdom.

In the meantime Santanu had  fallen in love with Satyavati, the daughter of a fisherman, who had become the mother of Vyasa from an encounter with Parasara Rishi, in which the Rishi had blessed her by changing the bad fishy smell of her  body into a heavenly fragrance. The fisherman accepted to consent to Satyavati’s marriage to the king, on the condition that her children would be the ones to ascend the throne. This excluded Ganga’s son from the succession. After some time, Devavrata himself went to talk with Satyavati’s mother and accepted his conditions, even swearing that he  would never have children of his own that could later claim succession rights against the descendants of Satyavati’s children. At that time the Devas bestowed on him the name of Bhishma.

The greedy scheme of Satyavati’s father actually brought about a disaster, because neither of her sons was able to continue the dynasty. Chitrangada (also called Chitraratha) was killed in battle when he was still very young, and Vichitravirya also died young before being able to beget sons. It is said he was afflicted by tuberculosis and a weak heart, so it is possible he was impotent.

Santanu finally retired to mount Archika in the Himalayas and attained liberation. His elder brother Devapi still lives in Kalapa waiting for the next Satya yuga when he will become king. In the meantime, Bhishma was the last remaining hero of the Kurus. He was asked to beget an heir, but he refused because he had taken a rigid vow as required by Satyavati’s father, so Satyavati called her early son, Vyasa, to give sons to Vicitravirya’s wives Ambika and Ambalika. The two women were terrified by Vyasa, so one closed her eyes and her son (Dhritarastra) was born blind and therefore unfit to ascend  the throne and continue the dynasty, while the other became pale with fear and her son (Pandu) was born with a very white complexion.

Pandu became the king and married Kunti and Madri; however because of a curse he could not have children and if he tried to have sex, he would  immediately die.

Before her marriage, Kunti had had a son (Karna) from Surya the Sun God, but because she was still very young and scared, she abandoned the baby on the river. However, on the request of Pandu she accepted to use her power to call the Devas and have children with them, so Yudhisthira was born from Yama (Dharma), Bhimasena was born from Vayu (like Hanuman, who is therefore his brother) and Arjuna was born from Indra.

Madri borrowed the mantra from Kunti and had Nakula and Sahadeva, the two twin sons of the Asvini kumaras. Pandu died and Madri followed him into death and to the higher planets.

Dhritarastra had also married, a princess from Afghanistan called Gandhari, who did not expect to get a blind husband but she bravely faced her fate by voluntarily making herself blind, too, so that her husband would not feel diminished in the relationship.

Gandhari became also pregnant but when she heard that Kunti had given birth to her first son before her, she became very angry and hit her own womb, and suffered a miscarriage. Vyasa came to help and divided the lump of flesh into 100 containers (like test tubes), from which Duryodhana and his 99 brothers were born.

At the death of Pandu, Dhritarastra had become the regent of the kingdom in the name of his minor sons, and he gradually became  attached to the idea of being the king. Also, he did not see why his son Duryodhana could not become king with full rights after him. So the old blind man and his sons, helped by the scheming uncle Sakuni (brother of Gandhari) tried many times to eliminate the five sons of Pandu by trying to murder them (by poison, fire, etc) and finally they sent them into exile, even depriving them of the kingdom of Indraprastha that the Pandavas had built for themselves with their own strength and with their volunteer followers. At the end of the exile term the Pandavas returned to claim the throne but Duryodhana refused, declaring that only war could settle the matter between the Pandavas and the Kauravas (as Duryodhana and his brothers considered themselves the only legitimate descendents of Kuru, the ancestor of Santanu).

The war could not be avoided, and as both Duryodhana and Arjuna had asked Krishna’s help. Krishna offered them the choice between his own great army of the Yadavas and himself unarmed and non-fighting. Duryodhana chose Krishna’s army and Krishna himself became the charioteer of Arjuna.