Dr. Shawqi Abu Khalil:
“The Jews of Khaibar contacted the people of the Ghatafan tribe, who were known to be mercenaries for hire. As a reward for fighting the Muslims, the Jews of Khaibar offered them a percentage of their yearly harvest, which consisted mainly of fruits and dates. They furthermore established alliances with the tribes of Fadak, Taima and Wadi Al-Qura; together, they were to launch a surprise attack on Al-Madinah. Having been informed of their plans, the Muslims who witnessed Al-Hudaibiyah travelled to Khaibar, in order to bring an end to the plotting of its inhabitants and their allies.” 
Shaykh Allamah Shibli Nomani
“Khaibar was the greatest centre of Jewish power in Arabia. Here did the chiefs of the Banu Nadir, when banished from Medina, take their abode, and work up the whole of Arabia against Islam, of which the first outcome was the battle of the Trenches. One of these chiefs, Huyayy Ibn Akhtab, was killed in the battle of Quraiza, and was succeeded by Abu Rafi, Sallam Abi al-Huquaiq who was a prosperous trader and a man of great influence.
The Ghatfan tribe, one of the most influential in Arabia, has its habitations adjacent to Khaibar and had always been in alliance with the Jews. In the sixth year of the Hijra, Sallam himself approached the Ghatfan and the other tribes in that region to mobilize them against the Muslims, and succeeded in raising a huge army to attack Medina. This activity was reported to the Prophet (p) and with his connivance Sallam was murdered, while asleep in his fort, by Abdullah Ibn Atik Of Khazraj, in the month of Ramadan, in the sixth year of the Hijra.
After Sallam, the Jews acknowledged Usair Ibn Rizam (some say it is Asir) to be their chief. He said to the Jews, assembled at a tribal meeting, that the measures taken by his predecessors were all useless; the best way was to attack the very capital of Muhammad (p) and that was the line he would allow.
Usair paid visit to the Ghatafan and other tribes and got a huge army ready. The Prophet (p) heard of it but not believing the rumours, he sent Abdullah Ibn Rawaha to Khaibar.
They met Usair and told him that the Prophet (p) had sent them with the proposal that, if he should present himself before him, the government of Khaibar would be given to him. Usair set out from Khaibar with an equal number of men. As a precautionary measure, this mixed caravan moved in twos, one Muslim rider being paired with a Jew. On reaching Qar Qara, Usair’s mind misgave him. He put out his hand to snatch away the sword from Abdullah Ibn Unais.
‘O enemy of God’, exclaimed Abdullah, ‘Do you wish to go back on your word?’ Saying that he urged his animal; and getting within the striking distance, cut his thigh with a stroke of the sword. Usair fell from his horse, but as he did so, he managed to wound Abdullah. The Muslims, now taking to offensive, fell upon the Jews with the result that all but one were killed. This took place towards the end of the sixth or the first month of the seventh year of the Hijra i.e., 628 C.E.
Now Khaibar had the most dangerous and the stiffest adversary. These were the people who had gone to Mecca and with the help of the Quraish stirred up a storm of revolt from one end of Arabia to the other. That storm had virtually rocked Medina, the very centre of Islam. The attempt had failed, yet the force at work were still operative. Of those who had been instrumental in bringing about the battle of the Trenches, the most influential had been the family of Ibn Abi al-Huquaiq. They belonged to the tribe of the Banu Nadir and had settled there when those people had been deported from Medina. They had taken possession of the famous fort of Qamus. Sallam Ibn Abi al-Huquaiq, already mentioned, had been the head of this family. His nephew, Kinana Ibn al-Rabi, Ibn Abi al-Huquaiq succeeded him as the head.
The Jews of Khaibar were hatching plots against Islam in collusion with the Ghatafan and also keeping themselves well-informed of the affairs at Medina through the hypocrites, who had always encouraged them to hope that the Muslims could never get the better of them.
The Prophet (p) had a wish to come to an agreement with these people, and with this end in view, he had sent Abdullah Ibn Rawaha. But the Jews were an unrelenting and a suspicious lot; moreover, the hypocrites Abdullah Ibn Ubbay Ibn Salul, sent them word that the Prophet (p) was planning an attack; but that they had nothing to fear, because the Muslims were a handful of men without arms, and did not count much. Having received this message the Jews sent Kinana and Hawdha Ibn Qais to the Ghatafan asking them to join them in the attack on Medina and promising to them half of the yield of the oasis. The Ghatafan according to a report, accepted these terms.
The Banu Fazara, an offshoot of the Ghatafan were very powerful. Hearing that the Jews of Khaibar were preparing for an attack against the Holy Prophet (p), they came to Khaibar and offered to fight the Muslims along with them. On receipt of this news, the Prophet (p) wrote to the Banu Fazara to desist from helping the Jews of Khaibar and promised to give them their share after the conquest of Khaibar. But the Banu Fazara refused.” 
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 Atlas of the Qur’an – Places. Nations. Landmarks. By Dr. Shawqi Abu Khalil, Page 307 – 308
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – Volume 2, page 156 – 159