Palestine had internationally recognized borders from 1922 to 1948. Right up until the day the Mandate was ended.
Depends upon one's point of view.
A widely internationally and formally recognized, autonomous and sovereign Lebanon existed on May 15, 1948.
A widely internationally and formally recognized, autonomous and sovereign Syria existed on May 15, 1948.
A widely internationally and formally recognized, autonomous and sovereign Transjordan existed on May 15, 1948.
A widely internationally and formally recognized, autonomous and sovereign Egypt existed on May 15, 1948.
No such widely internationally and formally recognized, autonmous and sovereign Palestine existed on May 15, 1948.
What existed in Old Palestine at the expiration of the British Mandate was a political vacuum bordered by legitimate and sovereign states.
The Mediterranean Sea and the borders of those preexisting sovereign states merely acted as the boundary lines for the maximum extent of the political vacuum.
The Jews of Old Palestine jumped-in quickly and claimed some of the land associated with that political vacuum as the basis for their own independent State.
Subsequent events made their claim a Reality.
Anyone not recognizing that Reality does, indeed, have recourse.
They may appeal the decision of history by force of arms.