The interpretation timeline
Proverbs 21:1 Douay-Rheims

As the divisions of waters, so the heart of the king is in the hand of the Lord: whithersoever he will he shall turn it.

A.D. 220 – 1875 5 witnesses 6 excerpts

Patristic

before A.D. 750
220
A.D.
Tertullian Patristic
c. A.D. 150–220
“This is the will of God: Look at the situation of the kingdoms and empires as arranged by God, in whose hand the heart of the king lies. Every day they plan for future income, from the registration of property, taxes in kind, gifts and taxes payable in money. But never up to this time has there been procured any such income by bringing the Christians under some sales tax for the person and the sect, when that could be a tremendous source of income because of our vast numbers, known to all. We are bought with blood, we are paid for in blood, we owe no money for our head, because Christ is our head. It is not fitting that Christ should cost us money. How could martyrdoms bring glory to God if by tribute we should pay for the liberty of our sect? And so, the one who bargains to have his freedom at a price goes counter to the divine dispensation.”
Source
“The phrase "Thy kingdom come" also refers to the same end as "Thy will be done," namely, [that God's kingdom may come] in ourselves. For, when does God not reign, "in whose hand is the heart of every king"? But, whatever we wish for ourselves, we direct our hope toward him, and we attribute to him what we expect from him.… This is the prayer of Christians; this shall bring shame to the heathens; this shall bring joy to the angels. It is for the coming of this kingdom that we are harassed now, or rather, it is for this coming that we pray.”
Source
373
A.D.
c. A.D. 296–373
“A desire to learn and a yearning for heavenly things is suitable to a religious emperor; for thus you will truly have "your heart" also "in the hand of God." Since then your piety desired to learn from us the faith of the catholic church, giving thanks for these things to the Lord, we counseled above all things to remind your piety of the faith confessed by the fathers at Nicea.”
Source
379
A.D.
Basil of Caesarea Patristic
c. A.D. 330–379
“If "the heart of the king is in the hand of God," he will not be saved through power of arms but through the divine guidance. Now, not any random person is in the hand of God but one who is worthy of the name of king.”

Post-Reformation

c. 1650 – 1900
1849
A.D.
1774–1849
“It. Though it be free, and may resist grace or embrace virtue, Isaias xlv. 22., Jeremias iii. 14., and vii. 3., and Josue xxiv. 23. (Calmet) — Yet God knows how to turn the heart even of a king, so as to preserve his liberty, with the same ease as a gardener brings the streams of water to his plants. (Menochius)”
1875
A.D.
Keil & Delitzsch Lutheran
1861–1875
“The group, like the preceding one, now closes with a proverb of the king. A king's heart in Jahve's hand is like brooks of water; He turneth it whithersoever He will. Brook and canal (the Quinta: ὑδραγωγοί) are both called פּלג, or פּלג, Job 20:17, Arab. falaj (from פּלג, to divide, according to which Aquila, Symmachus, and Theodotion, διαρέσεις; Venet. διανομαί; Jerome, divisiones); Jkt has the explanation of the word: "falaj is the name given to flowing water, particularly the brook from a spring, and every canal which is led from a spring out over flat ground." Such brooks of water are the heart of a king, i.e., it is compared to such, in Jahve's hand. The second line contains the point of comparison: He inclines it, gives to it the direction (הטּה, causat. of נטה, Num 21:15) toward whatever He will (חפץ denotes willing, as a bending and inclining, viz., of the will; vid., at Pro 18:2). Rightly Hitzig finds it not accidental that just the expression "brooks of water" is chosen as the figure for tractableness and subjection to government. In Isa 32:2, the princes of Judah are compared to "rivers of water in a dry place" with reference to the exhaustion of the land during the oppression of the Assyrian invasion; the proverb has specially in view evidences of kindness proceeding from the heart, as at Pro 16:15 the favour of the king is compared to clouds of latter rain emptying themselves in beneficent showers, and at Pro 19:12 to the dew refreshing the plants. But the speciality of the comparison here is, that the heart of the king, however highly exalted above his subjects, and so removed from their knowledge he may be, has yet One above it by whom it is moved by hidden influences, e.g., the prayer of the oppressed; for man is indeed free, yet he acts under the influence of divinely-directed circumstances and divine operations; and though he reject the guidance of God, yet from his conduct nothing results which the Omniscient, who is surprised by nothing, does not make subservient to His will in the world-plan of redemption. Rightly the Midrash: God gives to the world good or bad kings, according as He seeks to bless it or to visit it with punishment; all decisions that go forth from the king's mouth come לכתחלה, i.e., in their first commencement and their last reason they come from the Holy One.”
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.