Burger King was ready to play peacefully with its larger rival, until McDonald's fired back with a push for a bigger peace plan.

On Wednesday, Burger King began publicizing its idea for a "McWhopper" collaboration between the two chains, tied to Peace Day on Sept. 21.

Peace Day, as declared by the United Nations General Assembly, is meant to be "devoted to strengthening the ideals of peace, both within and among all nations and peoples."

Burger King's proposal, laid out at http://mcwhopper.com/, include a special sandwich, uniforms with both chains' logos and even a pop-up restaurant in Atlanta that would serve the special burger. Patrons wouldn't pay with money, but would instead pay by declaring their own truces on paper tray mats. Burger King said it picked Atlanta, because it is between its headquarters in Miami and McDonald's headquarters in Chicago (well, Oak Brook, Illinois, but BK put Chicago on its map).

The Burger King work was done by seven agencies including Alison Brod Public Relations, which sent the details out to reporters on Wednesday morning to spread the word. The other firms involved are WPP's Y&R New Zealand, Code & Theory, The David Agency, Rock Orange, Turner Duckworth and Horizon, the PR agency said.

McDonald's, in a short online letter simply signed "Steve, McDonald's CEO," said it thinks the brands "could do something bigger to make a difference." McDonald's, the world's largest restaurant chain by revenue, said it would be in touch with Burger King, and knocked down its rival's attempt to tie the marketing effort to Peace Day, without naming it directly.

"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," McDonald's said.