Final answer:
The Balfour Declaration became formal when it was adopted into the terms of the British Mandate for Palestine post-World War I and confirmed by the League of Nations, causing long-term Jewish-Arab tensions.
Step-by-step explanation:
The Balfour Declaration became a formal official document when it was incorporated into the terms of the British Mandate for Palestine. This occurred after World War I, as mandates were assigned at the San Remo conference in 1920 and subsequently confirmed by the League of Nations in 1922. The league's approval gave the declaration international recognition and legitimacy. The Balfour Declaration was initially a statement made in 1917 by British Foreign Secretary Arthur Balfour, expressing support for a 'national home for the Jewish people' in Palestine.
Although the declaration was a significant milestone for pro-Zionist groups, it also set the stage for increased tensions between Jewish and Arab populations in the region. This was due in part to conflicting promises made by the British, which also included supporting Arab nationalism. The complexities introduced by the declaration, and the British Mandate that followed, initiated a longstanding religious and territorial conflict that persists today.