Then on November 2, 1917, Arthur James Balfour wrote his letter to Lord Rothschild:
His Majesty's government view with favour the establishment in Palestine of a national home for the Jewish people, and will use their best endeavours to facilitate the achievement of this object, it being clearly understood that nothing shall 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.The vague wording of the Balfour Declaration might be compatible with Sykes-Picot's international regime for Palestine. But the agreement was secret, and unknown to the Zionists, who presumed Britain was free to make promises. (UPDATE: Martin Kramer notes in a comment that the Zionists knew about it from a leak, and links to his article.)
Then it all started to come apart. Only six days after the Balfour Declaration, this happened:
On November 23, both Pravda and Izvestia published Sykes-Picot. Three days later, The Manchester Guardian followed suit.
The cat was out of the bag. Britain denied it, but not very convincingly. Things were starting to unravel.