From History Net:
Convention Hall in Philadelphia, a mammoth art deco building on 34th and Spruce often used for prize boxing bouts, simmered in the glare of television lights as Republican delegates gathered there in the fourth week of June 1940 to choose their party’s candidate for president—and a plank on what, if anything, America should do about the war blazing in Europe.
The whiskey flowed freely, as at all such conclaves, but the war exerted a sobering influence on the proceedings. “Nazi fliers strike widely in Britain,” the New York Times reported in its June 25 edition, just three days after France’s formal surrender to Germany. Was it time, delegates asked themselves, to rally the party in favor of American intervention to put a stop to Hitler?
A full-page advertisement in that day’s Times, addressed to the convention-goers along with “American mothers, wage earners, farmers and veterans,” insisted the answer was no: “STOP THE MARCH TO WAR! STOP THE INTERVENTIONISTS AND WARMONGERS!” The missive was signed by a group calling itself THE NATIONAL COMMITTEE TO KEEP AMERICA OUT OF FOREIGN WARS.
Unbeknownst to the delegates, the ad was a propaganda plant, written by a German agent with close ties to Republican isolationists in Congress and paid for, in part, by the Nazi government in Berlin.