* Israel believed to be Middle East's sole nuclear armed power * U.S. and Israel see Iran as region's main proliferation threat * Say Arab initiative could undermine broader disarmament efforts By Fredrik Dahl VIENNA, Sept 25 (Reuters) - They may be joining forces in a new Middle East conflict, but Arab nations are defying the United States at a major meeting of the U.N. nuclear watchdog by trying to crank up pressure on Israel over its assumed atomic arsenal. Western diplomats say a vote on an Arab initiative to single out Israel for criticism at the Sept. ...