Burger King has pulled a surprise ad and posed a one-day ceasefire with its all-time rival McDonalds in order to support the non-profit group Peace One Day that aims to make this coming Sept 21 a literal "Peace Day" for all of them.

The giant chain wrote an open letter published in The New York Times and Chicago Tribune: "We'd like to propose a one-off collaboration between Burger King and McDonald's to create something special - something  that gets by the world talking about Peace Day."

The McWhopper.

All the tastiest bits of your Big Mac and our Whopper united in one delicious, peace-loving burger. Developed together, cooked together and available in one location for one day only -- Peace Day 2015, with all proceeds benefiting Peace One Day." This also came with a website and a proposal video.

However, McDonalds declined the offer "Inspiration for a good cause... great idea. We love the intention but think our two brands could do something bigger to make a difference."  

McDonald's CEO Steve Easterbrook further stated in Facebook that "between us there is simply a friendly business competition and certainly not the unequaled circumstances of the real pain and suffering of war." It seems the burger war was escalated.  "P.S. A simple phone call will do next time," he further added.

With the "settle the beef" offer was declined comes two other burger chains eyeing for the collaboration. The president of Wayback Burger John Eucalitto offered to take McDonald's place.

The Connecticut-based burger chain wrote: "Wayback Burgers would be happy to take Burger King up on its offer to partner to raise money for UNICEF surrounding International Day of Peace. Our nine-patty burger, The Triple Triple, could be a great vehicle to work with. Perhaps we could create the Whiple Whiple. Besides, wouldn't Burger King prefer to partner with a fresh and innovative brand that is on the rise? Either way, we hope to hear from you."

On the other hand, Denny's Diner, another restaurant chain with more than 1,600 branches across the USA, posted on their social media accounts and offered another hybrid burger. "hey @burgerking how about a Slampper© or a Whamper© or a Whoppaslamus-rex© or something?" as posted on their Twitter. As of the writing, Burger King hasn't responded to any of these new offers yet.