For centuries, the area that is now Israel had been controlled by the Ottoman Empire. In World War I, the Ottoman Empire took the side of Germany and the Central Powers. For this reason, it lost its possessions in the Middle East. After the war, the area that is now Israel was given to the British to administer as a League of Nations mandate.
Before the war was over, the British were thinking about what to do with that area of the world. They were also hoping to gain support from Jews all over the world for their war efforts. This might particularly be of help to them in areas like the Holy Land where the Jews could make trouble for the Ottomans.
In part for this reason, the British issued what is now called the Balfour Declaration in 1917. In this declaration, they committed themselves to the idea that a Jewish homeland should be created in the area that is now Israel. In doing so, they started the process by which Israel would eventually become an independent country controlled by Jews.