Khaybar was the northern power that had paid for the war against Madinah. Its chiefs had gone to Mecca after their own expulsion and put together the coalition that came to the Trench. Two years later the Prophet ﷺ went to settle it.
Sixteen hundred men marched, and the road chosen mattered more than the numbers: it brought the army in between Khaybar and Ghatafan. The oasis was rich enough to buy allies, and the camp at al-Rajiʿ sat squarely on the road those allies would have used.
The army arrived at night, and waited. It was his practice not to attack in darkness, but to listen at dawn for the call to prayer.