The British had the power to divide Palestine under the British Mandate for Palestine. The British drew up the basis for UNGAR 181- as they were legally permitted to do- and their, legal, resolution was submitted to- and passed, by majority vote in the UN.
Once the British abrogated their responsibilities to Palestine the border details of their- now democratically authorized- plan fell under the auspices of existing international law regarding borders. Those laws forbid the taking of territory by force and forbid annexation unless agreed by the contracting parties. Thus the delineations of 181 are protected under binding international law.
181 provided territory for two states and an area, Jerusalem, of international territory protected by law. Naughty Zionists wanted it all. Tch, tch. Illegal. Tough tittie.