The Balfour Declaration of November 2, 1917 was a recognition by the British Government that a national home for the Jews should be created in Palestine. But Palestine was never defined in the declaration. It was only a concept at the time. The territory of what was to become the British-awarded Palestine Mandate after World War I was under Ottoman rule at the time and was split into several sanjaqs or sub-provinces, one of which was the sanjaq of Jerusalem.
The territory of Palestine was not defined until September 1, 1922 as a line "drawn from a point two miles west of... [Aqaba] up the center of the Wadi Araba, Dead Sea and River Jordan to its juncture with the River Yarmuk; thence up the centre of the river to the Syrian frontier." This was the boundary between Palestine and Transjordan and, according to Suzanne Lalonde in Determining Boundaries in a Conflicted World "was approved by the League of Nations, and the language defining the boundary was actually incorporated within the text of the Palestine Mandate by decision of the League Council."1
Putting aside other agreements made before and after the Balfour Declaration (including the 1947 UN partition plan for Palestine, accepted by the Jews but not by the Arabs), the Balfour Declaration called for a Jewish national home in "Palestine" which later became defined, per above, as ending at the Jordan River. Which meant that West Bank was included within Palestine as so defined and therefore could be considered justified as an area where Jews could legitimately settle. This is not just an antique argument; I heard the Balfour Declaration recently cited by a Jewish interlocutor as a justification for Israeli settlements on the West Bank (which the Arabs claim as the area for a future Palestinian state.)
But the British Mandate also specified the following: ... "it being clearly understood that nothing should be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine, or the rights and political status enjoyed by Jews in any other country."
But how could a mass influx of Jews into Palestine not prejudice the "civil and religious rights" of existing non-Jewish communities there? The Balfour Declaration stands, along with the partition of India, as an icon to the micawberish policies of the British Empire at the start of its decline.
1Suzanne Lalonde, Determining Boundaries in a Conflicted World (Canada: McGill-Queen's University Press, 2002), p. 100.