Nothing, it seems, appeared to David more threatening in Absalom's plot than that Ahithophel was in it; for one good head, in such a design, is worth a thousand good hands. Absalom was himself no politician, but he had got one entirely in his interest that was, and would be the more dangerous because he had been all along acquainted with David's counsels and affairs; if therefore he can be baffled, Absalom is as good as routed and the head of the conspiracy cut off. This David endeavours to do.
I. By prayer. When he heard that Ahithophel was in the plot he lifted up his heart to God in this short prayer: Lord, turn the counsel of Ahithophel into foolishness, Sa2 15:31. He had not opportunity for a long prayer, but he was not one of those that thought he should be heard for his much speaking. It was a fervent prayer: "Lord, I pray thee, do this." God is well pleased with the importunity of those that come to him with their petitions. David is particular in this prayer; he names the person whose counsels he prays against. God gives us leave, in prayer, to be humbly and reverently free with him, and to mention the particular care, and fear, and grief, that lies heavily upon us. David prayed not against Ahithophel's person, but against his counsel, that God would turn it into foolishness, that, though he was a wise man, he might at this time give foolish counsel, or, if he gave wise counsel, that it might be rejected as foolish, or, if it were followed, that by some providence or other it might be defeated, and not attain the end. David prayed this in a firm belief that God has all hearts in his hand, and tongues too, that, when he pleases, he can take away the understanding of the aged and make the judges fools, (Job 12:17; Isa 3:2, Isa 3:3), and in hope that God would own and plead his just and injured cause. Note, We may pray in faith, and should pray with fervency, that God will turn that counsel into foolishness which is taken against his people.
II. By policy. We must second our prayer with our endeavours, else we tempt God. It is good service to countermine the policy of the church's enemies. When David came to the top of the mount, he worshipped God, Sa2 15:32. Note, Weeping must not hinder worshipping, but quicken it rather. Now he penned the third Psalm, as appears by the title; and some think that his singing this was the worship he now paid to God. Just now Providence brought Hushai to him. While he was yet speaking, God heard, and sent him the person that should be instrumental to befool Ahithophel. He came to condole with David on his present trouble, with his coat rent and earth upon his head; but David, having a great deal of confidence in his conduct and faithfulness, resolved to employ him as a spy upon Absalom. He would not take him with him (Sa2 15:33), for he had now more need of soldiers than counsellors, but sent him back to Jerusalem, to wait for Absalom's arrival, as a deserter from David, and to offer him his service, Sa2 15:34. Thus he might insinuate himself into his counsels, and defeat Ahithophel, either by dissuading Absalom from following his advice or by discovering it to David, that he might know where to stand upon his guard. How this gross dissimulation, which David put Hushai upon, can be justified, as a stratagem in war, I do not see. The best that can be made of it is that Absalom, if he rebel against his father, must stand upon his guard against all mankind, and, if he will be deceived, let him be deceived. David recommended Hushai to Zadok and Abiathar, as persons proper to be consulted with (Sa2 15:35), and to their two sons, as trusty men to be sent on errands to David, Sa2 15:36. Hushai, thus instructed, came to Jerusalem (Sa2 15:37), whither also Absalom soon after came with his forces. How soon do royal palaces and royal cities change their masters! But we look for a kingdom which cannot be thus shaken and in the possession of which we cannot be disturbed.

Turn the counsel of Ahithophel into foolishness - Ahithophel was a wise man, and well versed in state affairs; and God alone could confound his devices.

And one told David,.... That came either from Hebron or from Jerusalem:

Ahithophel is among the conspirators with Absalom; Absalom sent for him, and it seems he came to him, and continued with him, see Sa2 15:12,

and David said, O Lord, I pray thee, turn the counsel of Ahithophel into foolishness; either suffer him to give foolish counsel, or confound the schemes projected by him, and let them not be carried into execution; for God can, and sometimes does, disappoint crafty counsellors, that they cannot perform what they devise, but they are taken in their own craftiness, and their counsel is carried headlong, Job 5:12; this prayer was answered, Sa2 17:14.

David said, Turn, O Lord, . . . the counsel of Ahithophel--this senator being the mainstay of the conspiracy.