Christians of Najran

[AhmedGassama]

Assalamu alyakom

I was reading an article about the relationship of the prophet to the Christians of Najran, and i came to this point.

The story of the Najran people is not complete unless a later episode is mentioned; when the Prophet sent his military commander, Khalid ibn al-Walid, to the tribe of Al-Harith ibn Ka’b, in Najran. The Prophet ordered Khalid to call on those people to accept Islam and to give them a period of three days to make up their minds.
If they accepted Islam, Khalid was to accept that from them. If they refused, he would fight them. When Khalid arrived there, he sent his emissaries all over the place, calling on the people to accept Islam. They did so without much hesitation.
Khalid stayed there for some time to teach the people how to live according to Islam. He wrote to the Prophet about the results of his mission, and the Prophet wrote back asking him to return to Madinah bringing a delegation from that tribe. When they arrived in Madinah and spoke to the Prophet, he asked them: “How did you achieve your victories in pre-Islamic days?” They said: “We used to stick together and allow nothing to divide us into groups. We also never started any injustice.”
The Prophet said: “You are telling the truth.”

Now we all know that compulsion in Islam is unallowed, so how can the prophet ask Khaled to kill the christians if they don't convert ? This must be wrong, right ?
Can anyone of you tell me more about the subject if you know about the topic ?

And may Allah bless you!