Measured purely in terms of philanthropic bang-for-the-buck, the $4.3 million that the Rockefeller Brothers Fund has spent since 2003 on promoting a nuclear deal between America and Iran has to be one of the greatest bargains in the history of political charity.

That’s true regardless of whether America and Iran ultimately reach a nuclear deal, and regardless of whether you are in favor of such a deal or oppose it. The expenditure, exposed and detailed in a recent Bloomberg News article by Peter Waldman, amounts to less than the price of some two-bedroom apartments in Manhattan. It is a small fraction what Sheldon and Miriam Adelson spent supporting Newt Gingrich’s losing 2012 presidential campaign. Yet that sum has purchased a top item on President Obama’s foreign policy agenda, a directional change in American foreign policy toward Iran.

The left often sees Republican policy in an oversimplified model of being bought and paid for by the Koch Brothers or the Adelsons. Similar philanthropy on the left — George Soros, for example — sometimes attracts scrutiny, but it seems to be less often. The Rockefeller Brothers Iran policy spending is a reminder that spending by left-leaning mega-foundations deserves watching carefully, without falling into the trap of a reverse version of the “Kochtopus” fantasy.

The sometimes malevolent influence of Rockefeller money is, in one way or another, the subject of several recent books and one that is about to come out.

In The Tyranny of Experts, an NYU economist, William Easterly, traces the loss of China to the Communists back to a Rockefeller Foundation-organized meeting at the Yale Club in Manhattan in February 1925. The Rockefeller Foundation-funded Institute of Pacific Relations pursued a top-down, technocratic model of development. Easterly faults the Rockefeller Foundation for “blindness to the Chinese as individuals,” writing, “Conspicuously missing in Rockefeller’s discussion of China is any respect for the initiative and rights of the Chinese people themselves.”

James Piereson’s new book Shattered Consensus describes the Rockefeller Foundation, along with Ford, MacArthur, and Carnegie, as dwarfing conservative foundations in size, and pioneering a strategy of “advocacy philanthropy,” “designed to bring about large change by circumventing the electoral process.”

Even those on the Rockefeller payroll concede that results are sometimes mixed. Chef Dan Barber, in his book The Third Plate, quotes the Rockefeller family historian, Peter Johnson, describing the “green revolution” of super-productive wheat and rice crops developed by Rockefeller Foundation funded scientist Norman Borlaug as “a classic case of unintended consequences.” Barber credits the system of agriculture for saving a billion people from starvation — not bad! — but also blames it for being “disastrous for soil health,” for accelerating urbanization, and for reducing crop diversity.

To be sure, Rockefeller philanthropy has done plenty of unalloyed good. In a recent column, I credited a 1958 report of the Rockefeller Brothers Fund for setting the goal of 5% economic growth that John Kennedy ran on in his 1960 presidential campaign, and that Jeb Bush is echoing with his 4% growth goal.

But the left-leaning foundations can do plenty of damage, too. The Ford Foundation, with $12 billion in assets, recently announced it will shift its grant-making to focus on inequality. It was an ironical move for a charity where executive compensation reaches into the seven figures.

I’d rather this money be given away by those who earned it — even to foundations that exist long enough for the executives to drift far away from the original donors’ intentions — than it be taxed away by the government. The voluntary charitable sector is one of America’s great strengths, and its contributions enrich our national conversation. But one needn’t be a populist conspiracy theorist who sees the Koch Brothers “dark money” behind every small-government politician to suggest that when a non-profit foundation spends millions to change the federal government’s foreign or domestic policy, the expenditure itself deserves some public attention.

Mr. Stoll is editor of FutureOfCapitalism.com.