Why Israel Threatens To Bomb Iran

Tony Karon thinks it's merely part of a strategy to shape Western policy:

The centerpiece of Israel's Iran strategy has been to cultivate the belief that if sanctions and other pressures fail to force Tehran to yield, Israel will feel compelled to go to take military action, even without U.S. backing. Israel said nothing at all before its 1981 attack on Iraq's Osirak nuclear reactor, but scarcely a month has passed over the past three or four years without some new report calculated to create the impression that it was planning air strikes in Iran. The main line of criticism of Dagan in the Israeli camp did not challenge the content of what he said – that bombing Iran would be a catastrophic mistake, plunging Israel into a war it couldn't win but from there would be no exit; instead he was pilloried for giving the game away. 

Gary Sick concurs.