MIAMI & OAK BROOK, Ill. — History has it that during World Wat I, U.S. and German soldiers laid down their weapons temporarily for baseball games and Christmas celebrations. In the same spirit, Burger King has extended an olive branch to its fast-food arch-rival McDonald’s, calling for a one-day burger-war ceasefire in their longstanding rivalry for charity.

Burger King has proposed that on Peace Day, Sept. 21, 2015, the two quick-service restaurant (QSR) chains set aside their differences and unite to create the McWhopper, a burger that combines all the tastiest ingredients from their signature sandwiches—the Big Mac and the Whopper—in one delicious, peace-loving burger.

Peace Day is a United Nations-recognized annual day of global unity.

Burger King ran full-page ads in The New York Times and The Chicago Tribune to make the proposal.

The ‘McWhopper’ would be available for one-day, at one location in Atlanta—the approximate halfway point between Miami-based Burger King and Oak Brook, Ill.-based McDonald’s. The McWhopper restaurant, with a blended logo, would be staffed by crews for both companies, wearing hybrid uniforms.

“Our proposal is designed to generate the most attention and awareness possible for Peace Day and the work of the non-profit organization Peace One Day,” said Fernando Machado, senior vice president for global brand management at Burger King. “We’re being completely transparent with our approach because we want them to take this seriously. It would be amazing if McDonald’s agrees to do this. Let’s make history and generate a lot of noise around Peace Day. If they say no, we’ll hopefully have, at the very least, raised much-needed financial support and consciousness for the great cause that is Peace One Day. And both are well worth the effort.”

McDonalds quickly responded on Facebook:

Dear Burger King,

Inspiration for a good cause … great idea. We love the intention but think our two brands could do something bigger to make a difference. We commit to raise awareness worldwide, perhaps you’ll join us in a meaningful global effort? And every day, let’s acknowledge that between us there is simply a friendly business competition and certainly not the unequaled circumstances of the real pain and suffering of war. We’ll be in touch.

Steve, McDonald’s CEO

P.S. A simple phone call will do next time.

Burger King is now part of Oakville, Ontario-based Restaurant Brands International, created by the private-equity group 3G Capital when it merged the burger chain with Tim Hortons, a Canadian doughnut and coffee chain. McDonald’s is based in Oak Brook, Ill.