Complex sentence: The Declaration's protection against prejudicing the civil and religious rights of non-Jews proved to be untenable, having been put into effect via the British Mandate for Palestine which was confirmed in 1922 as the vehicle for delivering the promises of the Declaration.
Split into simpler sentences: The Declaration's protection against prejudicing the civil and religious rights of non-Jews proved to be untenable. The British Mandate for Palestine was confirmed in 1922 as the vehicle for delivering the promises of the Declaration.