Charles H. Spurgeon on Psalms 78:9

The children of Ephraim, being armed, and carrying bows, turned back in the day of battle. Well equipped and furnished with the best weapons of the times, the leading tribe failed in faith and courage and retreated before the foe. There were several particular instances of this, but probably the psalmist refers to the general failure of Ephraim to lead the tribes to the conquest of Canaan. How often have we also, although supplied with every gracious weapon, failed to wage successful war against our sins, we have marched onward gallantly enough till the testing hour has come, and then "in the day of battle "we have proved false to good resolutions and holy obligations. How altogether vain is unregenerate man! Array him in the best that nature and grace can supply, he still remains a helpless coward in the holy war, so long as he lacks a loyal faith in his God.