Because 6 months earlier, before the official date of the founding of Israel, Jewish paramilitary had started the ethnic cleansing of palastinians land.
The Arab country neighbours could well see the disaster that would unfold.
They had a right to defend their people.
Jewish "terrorisst" groups were operational in the year leading up to the the eventual end of the mandate and some form of independence. There was mutual violence ... none of it very edifying. The great non - event Arab army wasn't put into the field as a gallant attempt to libertae the poor Palestinians. Far from it. Jordan and Egypt were both in it for what they could get in terms of land and influence.
Had the Arabic war succeeded, the new slogan would have been, "From the river to the sea, Palastine is a newly acquired Jordanian province giving access to the Mediterranean Sea."
Not quite as catchy as the current model.
Like many / most significant military events, the 1948 / 49 war has a whole back story, full of disappointing nuance, paranoia, error and sheer greed. None of which sits comfortably within the need for clear heroes and villains that so many people seem to have. On balance, historians see the Arab venture as misguided, catastrophically executed and leaving a legacy of mistrust.
A perfidious Israeli plot it ain't.
