The sons of Reuben, &c. These three tribes, or at least so many of them as made a great army, joined their forces together, consisting of their best soldiers, to invade the country of the Hagarites. They were helped against them Against the Hagarites, who, it seems, fought stoutly; but God assisted the Israelites, enduing them with extraordinary courage and success, in consequence of their crying to him, and putting their trust in him, in his power, mercy, and faithfulness to his promise.
1 Chronicles 5:18-20
18 The sons of Reuben, and the Gadites, and half the tribe of Manasseh, of valiant men, men able to bear buckler and sword, and to shoot with bow, and skilful in war, were four and forty thousand seven hundred and threescore, that went out to the war.
19 And they made war with the Hagarites, with Jetur, and Nephish, and Nodab.
20 And they were helped against them, and the Hagarites were delivered into their hand, and all that were with them: for they cried to God in the battle, and he was intreated of them; because they put their trust in him.