Is "do one’s diligence and err on the side of caution" an idiom?

As Minty notes in a comment beneath the posted question, Mike Pompeo's quote uses two idiomatic phrases side by side: "to do diligence" and "to err on the side of caution." The latter is much more common in modern English than the former, but both may be found (in various permutations).

'To err on the side of caution'

The idiomatic phrase "to err on the side of caution" means "to take more precautions than are probably required, in order to ensure that one avoids some undesirable outcome." Two similar expressions, with much the same sense, are "to err on the side of safety" and to err on the side of vigilance"; In all three cases, the sense is that it is better to be overprepared for difficulties than to be underprepared. An expression that runs the other way is "to err on the side of optimism"—that is, to assume that things will go well and to mak no provision for dealing with serious and unexpected problems that might crop up.

'To do diligence'

The expression "to do diligence" is also an idiomatic phrase, but it is less common than "to err on the side of caution" and (viewed objectively) even stranger. To start with, diligence is normally understood as a characteristic, not an act. Here is the Merriam-Webster Online definition of the word:

diligence n 1 a : steady, earnest, and energetic effort : persevering application : ASSIDUITY {showed great diligence in tracking down the story} {working with diligence to scrub off the paint} b obsolete : speed, haste {Go, hence with diligence!— Shakespeare} 2 : the attention and care legally expected or required of a person (such as a party to a contract)

Whereas the definition 1a sense of diligence goes back to the fourteenth century; the definition 2 sense of the word dates only to 1742, according to Merriam-Webster (which as we shall see is off by at least 67 years). Neither of the two still-active meanings of diligence that the dictionary identifies goes very well with the verb do: Most English speakers wouldn't ask someone "Did you do steady, earnest, and energetic effort?" or "Did you do persevering application?" or "Did you do the [required] attention and care"?

Nevertheless, Chaucer does so in The Canterbury Tales (by 1400). For example, Saturne says this to his "dere doughter Venus" regarding her conflict with Mars, in "The Knight's Tale":

Min ben also the maladies colde, / The derke tresons, and the castes' olde: / My loking is the fader of pestilence. / Now wepe no more, I shal do diligence, / That Palamon, that is thin owen knight, / Shal have his lady, as thou hast him hight.

Here, it would seem, "do diligence" means simply "make every effort to ensure [some outcome]." This particular sense of "do diligence" seems to have dropped out of modern English.

A different group of English speakers used "do diligence" in a way that meant "do the [legally required] attention and care"—Scottish lawyers. And they did so at least as early as 1773 in connection with a notion of diligence as a count noun. From Marjory M'Pherson, Spouse to Edward M'Intosh, of Borland, Supplicant (January 18, 1773) in The Decisions of the Court of Session (Edinburgh, 1803):

She [Marjory M'Pherson] farther set forth that, about six weeks ago, the said Edward M'Intosh, her husband, on account of sundry diligences and captions against him, was obliged to leave his own house, and that part of the country where he used to reside, and has since absconded ; and there was the strongest reason to suspect that he intends to dispose of his estate, and to retire, with what price he can get for it, to some foreign country : But, in the mean time, his creditors are proceeding to diligence ; and, if by a subtlety of the law, the wife is debarred from a like course, both she and her child will be effectually excluded from the settlements and provisions in said contract of marriage : ... There may be other diligences carrying on ; but there is here sufficient ground for the court to authorise her to insist for implement and security of the provisions in her contract of marriage ; and therefore, praying the Court to authorise Donald M'Pherson of Breckacy, her father, a party-contractor in the contract of marriage, and captain Duncan M'Pherson, her brother, or either of them, to concur with her, or her daughter, and to do diligence, and to carry on processes against the said Edward M'Intosh, for implement and security of the provisions contained in the contract of marriage ; or to appoint such other persons to that effect, as to the Court shall seem meet.

What exactly were diligences and what did the Court of Session mean by "do diligence"? Black's Law Dictionary (1891/1968) explains:

DILIGENCE. ... In Scotch Law and Practice Process of law by which persons, lands, or effects are seized in execution or in security for debt [Citation omitted.] Process for enforcing the attendance of witnesses, or the production of writings. [Citation omitted.]

So when Mrs. M'Pherson (through her male relatives) sought to seize her absent husband's assets on the basis of a claim through a marital contract, she wanted the court's authorization to "do diligence" (pursue seizure of assets in compensation for a debt). That multiple such contractual claims to assets to repay debt were possible explains why there could be multiple "diligences."

The same 1803 collection of court cases includes several other instances of "do diligence." In John Cathcart against Hugh Mitchell and Others (July 26, 1775) and again in James Lamb and Others against James Duncan (May 16, 1798), the usage is much the same as in M'Pherson. But in Scrimzeour against Wedderburn (February 2, 1675), the sense of the expression seems quite different:

The Romans were liable to accept tutories [guardianships], ex lege, and therefore so soon as they knew of their nomination, they were obliged to act, unless within fifty days they obtained themselves to be excused, upon the excuses allowed in the law. But by our law no person is obliged to be a tutor, but by his own acceptance, and therefore cannot be liable for anything lost before his acceptance, not being obliged to do diligence till he accepted ; ...

Here the sense of "do diligence" seems to be "perform the responsibilities that come with accepting a particular role or status under law." This is very similar to Merriam-Webster's definition 2 of diligence—the attention and care legally expected or required of a person—used 67 years before the earliest instance date that MW cites.

An even earlier instance of "do [one's] diligence" in the sense of "do one's duty" appears in "The Confessioun of John Habroun, young Talla, Dagleish and Powrie, upon quhom was Justice execute the 3d of Januarie, the yeare, the yeare of God 1567," related to the murder of Lord Darnley, reprinted in Malcolm Laing, The History of Scotland, from the Union of the Crowns, on the Accession of James VI to the Throne of England, to the Union of the Kingdoms in the Reign of Queen Anne, volume 2, second edition (1804):

Quhairfoir let all men flee evill cumpany, and to trust not in men, for redy are we to imbrace evil, as redy as hardes to receive fyre. And further in the Tolbuith he requirit John Brande, minister of the congregation, to passe to my lord Lindsay, and say, my lord, hartily I forgeve your L. and als my lord regent, and all others, but specially them that betrayit me to you, for I knowand all others, but specially tham that betrayit me to you, for I know if ye could have savit me ye would, desiring you, as ye will answere before God in the latter day, to do your diligence to bring the rest quho was the beginners of this orke to justice, as ye have done to me, for ye know it was not begunne in my head, but yit prayses God that his justice has begunne at me, by the quhilk he hes callit me to repentaunce.

Likewise, from "Diligence," in David Lindsay, Ane Satyre of the Thrie Estaits: In Commendation of Vertew and Vitvperation of Vice (1602):

Famous peopil, hartlie I ȝow requyre / This lytill sport to tak in patience. / We traist to God, and we leif ane vther ȝeir, / Quhair we haue failit, we sall do diligence, / With mair pleasure, to mak ȝow recompence; / Because we haue bene, sum part, tedious, / With mater rude, denude of eloquence, / Likewyse, perchance, to sum men odious. 'Do diligence' and 'due diligence'

From the Scottish knight's oath in John Selden, Titles of Honor, second edition (1631):

7. Ye sall do diligence quhaire euer ye heir thair are any Murtherars, Traytors or Maisterfull Theiues and Rauaris that oppreseth his Maiefties Ledges and poore, to bring them to to the Lawes or Iustices with diligence at all your power.

And from John Skene, De Verborum Significatione: The Exposition of the Termes and Difficill Wordes, Conteined in the Foure Buiks of Regiam Maiestatem (1641):

RECOGNITION of landes is commonly vsed in the Law, and practique of this realme. ... And not understanding that the saising is taken bee the superiour, zit the vassall or possessour tinis nor forefaltis na waies the propertie of the saides landes, untill zeir and daie be out-run : Swa that he do diligence within fortie daies after the said recognition, and taking of the saising, to crave and ask fra his superiour the saides landes to him to borgh, that is, to repledge them, find and pledge and caution, that he sall be reddie to do to his superior, anente the saides landes, all that equity and lawe requiris.

Interestingly, Scottish law seems to have preserved the old notion of diligence as a pursuit of debt or other obligation, at least in some situation. From a model clause on "Diligence" in United Kingdom Mineral & Mining Sector Investment and Business Guide:

35. If and when any of the payments mentioned in clause 8(1) of this license or any part thereof shall be in arrear or unpaid for 28 days, next after any of the days whereon the same ought to be paid (whether the same shall have been legally demanded or not) then and so often as the same may happen the Minister may (as an additional remedy and without prejudice to any other rights and remedies to which he would be entitled) do diligence in respect thereof in like manner as a landlord may do diligence in respect of unpaid arrears of rent and such diligence shall be effectual to attach all or any of the stocks of petroleum, engines, machinery, tools, implements and other effects belonging to the Licensee which shall be found on or about any of the Licensee's installations and equipment used or to be used in connection with searching, boring for or getting petroleum in the licensed area, ..."

'Do diligence' and 'due diligence'

Black's Law Dictionary devotes considerable space to discussing diligence in the (non-Scottish) legal sense of an active obligation that a person must perform as part of possessing a particular status under law. This type of diligence constitutes very nearly the mirror opposite of negligence, there being, under both English common law and civil law, three levels of each: common or ordinary, high or great or extraordinary, and low or slight. Because different levels of diligence applied to different legal relationships, it was necessary to speak not merely of "diligence" but of "due diligence"—the level of diligence appropriate in a particular relationship under review by the court.

A Google Books search finds instances of "due diligence" from as early as 1606. From Thomas Fitzherbert, The First Part of a Treatise Concerning Policy, and Religion (1606):

16. To conclude this aduise, one general rule is to be held, that to temporize or delay time is neuer good but in three cases. The first when matters are not maturely consulted and wel digested (wherin neuertheles al due diligence is to be vsed, lest time and opportunity of action, be spent and lost in consultation)[.]

And from a 1607 translation of An Apology, or, Apologiticall Answere, made by Father Paule a Venetian ... (1607):

But I would not now haue any man deceiued by that which the Author adds, Viz. That in a doubtfull case a man is to follo the the iudgement of the superiour, and not his owne. For a case may be said to be doubtfull in two sences : either doubtfull, wherein a man hath not indeuored to resolue himselfe ; or doubtfull, wherein after his due diligence imployed, he cannnot yet resolue himselfe.

The more common verb used with "due diligence" is use, not do, as this Ngram chart of "use due diligence" (blue line) versus "uses due diligence" (red line) versus "used due diligence" (green line) versus "do due diligence" (yellow line) versus "does due diligence" (dark blue line) versus "did due diligence" (pink line) for the period 1750–2005 indicates:

Still, it is noteworthy that "do due diligence" has, since 1990 or so, grown significantly more common, even surpassing "used due diligence" as the second most common of the six expressions tracked, as of 2005.

The earliest Google Books matches for "do due diligence" are again associated with a Scottish Court of Session decision. From Patrick Pearson versus William Jack (June 28, 1825) in Cases Decided in the House of Lords: On Appeal from the Courts of Scotland (1829):

The Court of Session having in general found a party guilty of various charges made against him in a petition and complaint; a remit made ex parte to review the judgment, and state in what respects he had been guilty.

The Lord Ordinary, on the 13th June 1822, pronounced this interlocutor:—'In respect that a demand is no made by the pursuer [plaintiff], for having the defender found liable for the whole sum of L.169, contained in the bill by Jack an Meldram, and that the defender alleges that he is not liable for that debt, in consequence of his having done due diligence against the parties by whom it was due ; appoints the defender to give in a condescendence, in terms of the Act of Sederunt, of the facts and circumstances he avers and offers to instruct, in order to shew that such diligence was done by him ; and to accompany said condescendence with production of any execution of the said diligence against Jack which may be in possession.'

From Cairn's Trustees versus William Brown in Session Cases: Cases Decided in the Court of Session (1836):

3. He [Brown, who drew up a £200 bill of loan from Cairns for the benefit of Thomson, who later declared bankruptcy] is liable also on the ground of culpable negligence in not enforcing payment from Thomson, inasmuch as he neglected to do due diligence in recovering the contents of the bill for many months after it had, at his own request, been put into his hands for that purpose, having allowed six months before Thomson's bankruptcy and nine months after that event to elapse, without taking any step to recover the contents of the bill.

And from Mrs. Katherine Augusta Ritchie or Lambe and Husband and Others v. Mrs. Mary Stuart Chapman or Ritchie and Husband and William Carmichael and George Lang (December 22, 1837) in Reports of Cases Decided in the Supreme Courts of Scotland (1838):

The pursuers [plaintiffs] pleaded— ... (4.) That the defenders are liable for all sums for the recovery of which they cannot show that the factrix [estate agent] did due diligence.

Conclusion

As the foregoing discussion indicates, several interconnected terms involving diligence occur in English: "do diligence" (exert efforts that are legally required or otherwise deemed appropriate), "due diligence" (a standard of attention or care appropriate for a certain obligation or responsibility), and "do due diligence" (a combination of the first two terms).

The notion of "doing diligence" as a matter of legal responsibility seems to have arisen first in Scottish law, where diligence could refer either to a standard level of care and attention or to a specific effort to obtain payment of a particular debt or other obligation. Today the idea of "doing diligence" or "doing due diligence" still has legal overtones, but it can also be used in a nonlegal sense to mean "take proper precautions to avoid entering into a dangerous situation or bad agreement (contractual or otherwise)." In particular, "doing [due] diligence" refers to making appropriate inquiries and conducting a suitable background investigation to ensure that the business or political result of an agreement will not be a debacle.

The expression "do [due] diligence" might therefore, with equal validity, apply to researching candidates for public office within a government before actually nominating them or to doing a background check before entering into a business relationship, as in this example from Seth Hettena, Trump/Russia: A Definitive History (2018):

"It's not very hard to get connected to Donald if you make it known that you have a lot of money and you want to do deals and you want to put his name on them," Abe Wallach, who was the future president's right-hand man at the Trump Organization from 1990 to about 2002, told Bloomberg. “Donald doesn't do due diligence. He relies on his gut and whether he thinks you have good genes."

Likewise, from Robert Brunner, ‎Stewart Emery & ‎Russ Hall, Building a Design-Driven Corporate Culture (2009) [combined snippets]:

What's really important is that you have to do diligence when you're developing something. You have to understand what the potential risks and potential failures are, and the potential impact to your company.

Such is evidently the meaning of the phrase in Mike Pompeo's comment, whether he actually said "Any nation or entity interacting with Iran should do its diligence..." or "Any nation or entity interacting with Iran should do its due diligence..."