Before the Balfour Declaration, the British promised the region to the Arabs if they helped fight against the Ottoman Empire via the McMahon–Hussein correspondence:

https://en.wikipedia.org/wiki/McMahon%E2%80%93Hussein_corres...

Basically, the region was double promised to both sides during WWI, Arabs in the region being promised it first. This crucial detail is somehow often left out of any discussion regarding the Balfour Declaration even though it preceded it.