The Prophet (p) sent an expedition to the King (or Governor) of Dumatul Jandal. The reason he sent Khalid Ibn Waleed (ra) to him, was because this Christian King was in alliance with Byzantine (Roman) Empire. The Byzantine Empire had declared war against the Muslims. Which we have written about before (here). This King backed The Byzantine Empire financially and militarily. Hence, Khalid Ibn Walid captured this King and brought him to the Prophet (p), they made a peace treaty.
“The Prophet (p) sent Khalid ibn al-Walid to Ukaydir of Dumah. He was seized and they brought him to him (i.e. the Prophet). He spared his life and made peace with him on condition that he should pay jizyah (poll-tax).” (Abu Dawud Book 19, Number 3031)
Dr. Mustafa As-Sibaa’ie:
“ARRIVING AT TABOOK
When the Prophet arrived at Tabook, he found not even trace of the large armies of Rome and of loyal Arab Christian tribes. The Prophet certainly gave them ample time to bring their forces to Tabook, for he made camp there for twenty nights; but even had he stayed longer, they still would not have come. For the Romans were forced to show that they had no desire to enter into battle against the Muslims; and even Arab Christian tribes preferred to lay low while the Prophet was at Tabook.
As for the rulers of cities that were scattered all along the borders of Ash-Sham (Syria and surrounding regions), they decided to surrender peacefully, to sign peace treaties, and to promise to pay the Muslims a mandatory head tax the Jizyah; for example, the king of Ailah sent gifts to the Prophet a while mule and a robe and agreed to pay the Jizyah.
Ukaidir, the king of Daumatul-Jandal, also signed a treaty with the Muslims and agreed to pay the Jizyah tax. But his case was different in that he did not voluntarily go to the Muslims; instead, he made up his mind to make peace with them after he was captured by them.
The Prophet had sent out a unit of four-hundred and twenty horsemen to Daumatul-Jundal. Headed by Khaalid ibn Waleed, the men of the unit managed to capture Ukaidir ibn Abdul-Malik Al-Kindee, who was out of the city on a hunting excursion. It was after Ukaidir was taken to the Muslims that he agreed to pay the Jizyah tax. …” 
Shaykh Allamah Shibli Nomani:
“At Dumat Al-Jandal he presented a white donkey which brought to him the gift of the Prophet’s garment in return. The Christians of Jarba and Adhruh attended likewise and consented to the levy of the Jizya. At Dumat al-Jandal (also pronounced as Daumat al-Jandal), which is five stages from Damascus, there was an Arab chief, Ukaidir by name, who owed allegiance to the Roman Emperor.
Khalid Ibn Walid was despatched with four hundred and twenty men to subdue him. Khalid made captive, and later on released him on condition that he would personally appear before the Prophet (p) to settle terms. Accordingly, he arrived accompanied by his brother and was promised protection.” 
 The Life Of Prophet Muhammad: Highlights And Lessons by Dr. Mustafa As-Sibaa’ie, page 1439 – 1440
 Sirat -un- Nabi [Sallallahu Alaihi Wasallam] By Shaykh Allamah Shibli Nomani (r.a), volume 2, page 238