So this seems to be one of those events that only gets brief mentions here and there and then gets talked about more in later literature.
The person who invaded the Sakyan republic was Pasenadi’s son, Viṭaṭūbha. You can ready about what happened here: Viṭaṭūbha
Basically the Buddha tried a kind of non-violent protest, but it didn’t work. The Sakyans were slaughtered.
Edit, here’s the entry, its from The Dictionary of Pali Proper Names by G.P. Malalasekera:
v.l. Viḍūḍabha.– Son of Pasenadi and Vāsabhakhattiyā. On the birth of Viḍūḍabha, the king, glad at having a son, sent word to his own grandmother asking her to choose a name. The minister who delivered the message was deaf, and when the grandmother spoke of Vāsabhakhattiyā as being dear to the king, mistook “vallabha” for “Viḍūḍabha,” and, thinking that this was an old family name, bestowed it on the prince. When the boy was quite young, Pasenadi conferred on him the rank of general (senāpati), thinking that this would please the Buddha.¹
When Viḍūḍabha was seven years old, he wished to visit his maternal grandparents, hoping to be given presents, like his companions by theirs, but Vāsabhakhattiyā persuaded him against this, telling him that they lived too far away. However, he continued to express this desire, and when he reached the age of sixteen she consented to his going. Thereupon, accompanied by a large retinue, he set out for Kapilavatthu. The Sākyā sent all the younger princes away, there being thus none to pay obeisance to him in answer to his salute, the remaining ones being older than he. He was shown every hospitality and stayed for several days. On the day of his departure, one of his retinue overheard a contemptuous remark passed by a slave woman who was washing, with milk and water, the seat on which Viḍūḍabha had sat. This was reported to him, and, having discovered the deceit which had been practiced on his father, he vowed vengeance on the Sākyā. Pasenadi cut off all honours from Vāsabhakhattiyā and her son, but restored them later, at the Buddha’s suggestion.
After Pasenadi’s death, which was brought about by the treachery of Dīghakārāyana in making Viḍūḍabha king,² Viḍūḍabha remembered his oath, and set out with a large army for Kapilavatthu. The Buddha, aware of this, stood under a tree, with scanty shade, just within the boundaries of the Sakyan kingdom. On the boundary was a banyan which gave deep shade. Viḍūḍabha, seeing the Buddha, asked him to sit under the banyan. “Be not worried,” said the Buddha, “the shade of my kinsmen keeps me cool.” Viḍūḍabha understood and returned home with his army.³
Three times he marched against the Sākyā and three times he saw the Buddha under the same tree and turned back. The fourth time the Buddha knew that the fate of the Sākyā could not be averted and remained away. In a previous existence they had conspired and thrown poison into a river.
The Sākyā went armed into the battle, but not wishing to kill, they shot their arrows into Viḍūḍabha’s ranks without killing anyone. On this being brought to Viḍūḍabha’s notice, he gave orders that all the Sākyā, with the exception of the followers of the Sakyan Mahānāma, should be slain. The Sākyā stood their ground, some with blades of grass and some with reeds. These were spared, and came to be known as Tiṇasākiyā and Naḷasākiyā respectively.⁴ The others were all killed, even down to the infants. Mahānāma was taken prisoner and went back with Viḍūḍabha, who wished him to share his meal. However, Mahānāma said he wished to bathe, and plunged into a lake with the idea of dying rather than eating with a slave woman’s child. The Nāgā of the lake, however, saved him and took him to the Nāga world. That same night Viḍūḍabha pitched his camp on the dry bed of the Aciravatī. Some of his men lay on the banks, others on the river bed. Some of those who lay on the river bed were not guilty of evil in their past lives, while some who slept on the bank were. Ants appeared on the ground where the sinless ones lay, and they changed their sleeping places. During the night there was a sudden flood, and Viḍūḍabha and those of his retinue who slept in the river-bed were washed into the sea.⁵
Footnotes
¹ It was for the same reason he married Vāsabhakhattiyā; both in the Piyajātika Sutta (M.ii.110) and the Kaṇṇakatthala Sutta (M.ii.127) Viḍūḍabha is spoken of as a general (senāpati).
² For details see Pasenadi.
³ This exposure to the sun gave the Buddha a headache that lasted throughout his life (UdA.265; Ap.i.300).
⁴ According to Chinese records, Viḍūḍabha took five hundred Sakyan maidens into his harem, but they refused to submit to him and abused him and his family. He ordered them to be killed, their hands and feet to be cut off, and their bodies thrown into a ditch. The Buddha sent a monk to teach them, and they were reborn after death in heaven. Śakra collected their bones and burnt them (Beal, op.cit. ii.11 f ). The eleventh Pallava of the Avadānakalpalatā has a similar story. Viḍūḍabha killed seventy-seven thousand Sākyā and stole eighty thousand boys and girls. The girls were rude to him, and he ordered their death.
⁵ This account is taken from DhA.i.346‑9, 357‑61; but see also J.i.133 and iv.146 f, 151 f.
So it seems the main source here is the *Dhammapada Aṭṭhakathā
However, I will also mention that the event is also known in Northern Buddhist literature (see note 4). Not sure what the source is in the northern tradition though, maybe someone else can speak to it? @cdpatton ?
Also, a Tibetan Buddhist lama I know has mentioned this events various times, so it seems it is well known there too.
