1 Samuel 30:10
But David pursued, he and four hundred men: for two hundred abode behind, which were so faint that they could not go over the brook Besor. But David pursued, he and four hundred men: for two hundred abode behind, which were so faint that they could not go over the brook Besor.
This belongs to David's victory over the Amalekites before he became king, a victory soon followed by his accession to the kingship of Judah. Of his six hundred men, David continued the pursuit of the Amalekites with four hundred while two hundred had to stop and remain at the brook Besor. Those two hundred stayed because they were too faint, weary, and exhausted to go farther and to cross the brook. They had gone as far as their strength allowed and did not stay behind merely to avoid the fight.
Those left behind were set to guard the baggage, supplies, and equipment, holding the base while the others went forward. Their remaining behind was with David's consent.