What Is the Balfour Declaration?

The Balfour Declaration was signed by the British in 1917. It said if you break away from the Ottoman Empire we will give you an Arab chelaffa spanning from Baghdad to Yemen. This is a public promise Everybody knew it and so the Shri Farmaka declared war against the Ottomans. They were out of Jews living in peace and harmony no problems in that region So how could this letter come about that is saying in this 3% Jewish land? We're gonna make it an entire Jewish land?