When Israel was created as independent nation, its neighbors in the Middle East reacted militarily. Why?

The roots of this conflict go right through history. There
is a religious element as the Jews believed it was a land to which they were destined to
return. Also, it is religiously significant for Jews, Muslims and Christians.  However
it is wrong to overplay the religion card. Primarly the conflict was initiated over
geopolitical considerations.

WW1 was key to creating the
tension. In the Balfour Declaration the British, pressured by Weizmann, promised the
Jews a homeland within the region known as Palestine (it should be noted that Palestine
isn't, and never was, a recognised state- it is simply an area). In addition to this in
the McMahon-Husseyn correspondence the British also promised the Arabs limited sovereign
control of the area if they rebelled against their Ottoman rulers. Thus, both sides
believed they had a rightful claim to the area.

When Israel
was officially created the Arabs felt betrayed. From their perspective they had lived
there continuously for hundreds of years, and they had been promised sovereignty. The
Arabs nations who attacked did so out of geopolitcal considerations. Israel was viewed
as an imperial force and the initial conflict was a result of this. Religion is a
factor, but typical geopolitical considerations took
precedence.

*This offers an unreferenced
summary. Research into the declarations mentioned, the British Mandate and the UN
mandate should provide a clearer explanation*