The refugees ghave fled fripon the war, not specifically from Assad. If a negotiated peace can include a guarantee of free elections, properly monitoored, with the refugees having votes, that could ensure a safer envirobment allowing people to return home.
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In no way did the UN "give" Israel most of the territory it hold on to today.

In any case, if the UN were to "give" Hertfordshire to, say, exiled Syrians, to set up a new country, would you regard that as a fair title of ownership? Palestine didn't belong to the UN, any more than it belonged to Britain when the Balfour Declaration was issued.