It is claimed by some that the third ‘expedition’ against Wadi al-Qura took place because the Muslims wanted to ‘conquer their land’. This is not true.
According to authentic Islamic sources, the Muslims were sent merely to preach Islam, nothing else. While they were preaching the message of Islam peacefully in Wadi al-Qur the Jews started hostilities against the Muslims and attacked them. As a result of the hostilities and attacks against Muslims, a battle ensued. And they lost.
I would like to add, in one report it is mentioned that these people were also in alliance with an enemy, providing them with money against the Muslims.
Kitab Futuh al-Buldan:
“When the Prophet departed from Khaibar, he came to Wadi-l-Kura and invited its people to Islam. They refused AND STARTED HOSTILITIES. The prophet reduced the place by force…” 
“When the Messenger of God came to al-Sahba he went to Birma until he came to Wadi al-Qura seeking those Jews who lived there. Abu Hurayra related: We set out with the Messenger of God from Khaybar to Wadi al-Qura. Rifa’a b. Zayd b. Wahb al-Judhami had gifted to the Messenger of God. When they alighted to Wadi al-Qura, we finally reached the Jews and the people from the Bedioun had recourse to them. While Mid’am settled the Messenger of God, the Jews received us with spears where we alighted. There was no preparation and they were shouting in their fortresses. A destitute arrow pierced Mid’am and killed him. The people said: Paradise will delight you. …” 
“We were with the Messenger of God in the year of Khaibar … The Messenger of God set out for Wadi Al-Qura. When we were in Wadi Al-Qura, while Mid’am was unloading the luggage of the Messenger of God, an arrow came and killed him. …” Sunan an-Nasa’i vol. 4, Book 35, Hadith 3858
Muwatta Imam Malik:
“We went out with the Messenger of God, in the year of Khaybar. … The Messenger of God, made for Wadi’l-Qura, and when he arrived there, Midam was unsaddling the camel of the Messenger of God, , when a stray arrow struck and killed him. …” (Muwatta Malik Book 21, Hadith 25)
Shaykh Allamah Shibli Nomani
“Having reduced Khaibar, the Prophet (p) started for Wadi al-Qura. He did not mean to fight, but the Jews were prepared beforehand, and began a shower of arrows. Mud’im in the service of the Prophet (p), who was taking off the saddle from the camel’s back was struck and died. Historians generally do not speak of the preparations made by the Jews. But Imam Baihaqi had definitely said,
‘The Jews came out to meet us with arrow-shots, for which we were not ready.’ 1 (Zurqani, on Mo’atta, borrowing from Baihaqi, chapter al-Jihad and discourse of Ghulul, p. 313)
Anyhow, a battles ensued. After a brief fight the Jews surrendered and peace was made on the terms of Khaibar.” 
Safi ur Rahman Al Mubarakpuri:
After Khaybar had been subjugated, the Prophet went to Wadi Al-Qura and invited the Jews there to accept Islam. Rather than accept or enter a pact, they took the military option. The first of their champions came out and was slain by Zubayr. The second one who came out met the same fate. Ali slew the third. In this way, the Jews lost eleven men. At the death of each, the Prophet would repeat his call to Islam, and at every prayer he would so the same. The day ended thus. The following morning he appeared again, but before the sun climbed any higher the Jews were routed, with a sizable amount of land falling into Muslims hands.
It was time to sue for peace, and the Jews asked to be given the same status as the people of Khaybar. Their request was granted, and yet another area came under Muslim protection.” 
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.” 
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 The origins of the Islamic state, being a translation from the Arabic, accompanied with annotations, geographic and historic notes of the Kitab futuh al-buldan of al-Imam abu-l Abbas Ahmad ibn-Jabir al-Baladhuri, [Published 1916], volume 1, page 57
 The Life of Muhammad: Al-Waqidi’s Kitab Al-Maghazi, page 349
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a) – volume 2, Page 180
 When The Moon Split By Safi ur Rahman Al Mubarakpuri, page 237
 Atlas of the Qur’an – Places. Nations. Landmarks. By Dr. Shawqi Abu Khalil, Page 307 – 308