Two billionaires on opposite ends of the political spectrum, left-wing financier George Soros and right-wing industrialist Charles Koch, are to finance a new think tank which its co-founders say will challenge America’s military-industrial complex by promoting an end to “endless war” abroad.

The Quincy Institute for Responsible Statecraft will launch in September, having received an initial $500,000 (£385,000) from Mr Soros and Mr Koch apiece. The Boston Globe said a handful of other donors had contributed an additional $800,000 to the project.

The name is an homage to John Quincy Adams, a former US president who said in a speech in 1821 that the US "goes not abroad in search of monsters to destroy. She is the well-wisher to the freedom and independence of all. She is the champion and vindicator only of her own".

At first sight, Mr Soros and Mr Koch – two of the wealthiest men on Earth - would appear an extraordinarily unlikely partnership for such an enterprise.

The former, a lifelong progressive, runs the second biggest philanthropic organisation in the world, the Open Democracy Foundations, and is hated by much of America's far-right, many of whom promote various antisemitic conspiracy theories in an attempt to discredit the Hungarian-American businessman.

Charles Koch, a driving force behind the modern Republican Party, has for decades pushed for increased deregulation and smaller government. He is a founder of the Tea Party movement and one of the leading proponents of climate change scepticism. Of Mr Soros’s left-leaning views, Mr Koch once said he had not “been sufficiently exposed to the ideas of liberty”.

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The Quincy Institute's website, however, states the "action-oriented" think tank will promote a foreign policy centred on "diplomatic engagement and military restraint" supported by "like-minded progressives and conservatives" - perhaps a nod to the involvement of two businessmen often at odds politically.

In an editorial for the Globe, historian Stephen Kinzer, who spoke to the co-founders, said the foundation was likely to advocate for a return to the 2015 Iran nuclear deal, for the withdrawal of US troops from Syria and Afghanistan, and for less confrontational approaches to perceived adversaries such as Russia and China.

Announcing the launch, Stephen Wertheim, one of five co-founders of the Quincy Foundation, said on Sunday the think tank will “proceed from the premise that foreign policy should secure the safety and well-being of the American people while respecting the rights and dignity of all”.

Texas Republican congressman Louise Gohmert claimed George Soros helped the Nazis

“The Quincy Institute will have experts of its own and provide a platform for the expertise of others, but it will also recognise that the American people have the right and responsibility to be the ultimate arbiters of US foreign policy,” the Columbia University historian said.

The US, Mr Wertheim tweeted, keeps on “inflicting violence” on people through its interventionist foreign policies, which benefits neither the victims nor American citizens, other than a “handful of massive 'defence' companies”.

“American foreign policy is in crisis — a crisis of morality and strategy alike. The crisis goes well beyond the current occupant of the White House and afflicts both political parties,” he added. “If we are serious about building a new, 21st-century foreign policy, centred on diplomatic engagement and military restraint, we need to work together to take on the military-industrial complex.”

Mr Wertheim’s co-founders are author and journalist Eli Clifton; Suzanne DiMaggio, a senior fellow at the Carnegie Endowment for International Peace; Trita Parsi, an academic and founder of the National Iranian American Council; and Andrew Bacevich, an international relations historian at Boston University.

All have been notable critics of US foreign policy in recent years.

Mr Clifton on Sunday tweeted: The [Quincy Institute] promotes ideas that move US foreign policy away from endless war and toward vigorous diplomacy in the pursuit of international peace."

Mr Bacevich, asked why he was co-founding the organisation, said: “The Quincy Institute will invite both progressives and anti-interventionist conservatives to consider a new, less militarized approach to policy.

“We oppose endless, counterproductive war. We want to restore the pursuit of peace to the nation’s foreign policy agenda.”