A unit of the Iron Dome anti-rocket system will take up position in the Negev this morning for what is being called an "operational trial" outside the laboratory, facing actual threats. The system, however, will still not be at full capacity. Full capacity is needed to declare it operational. The early deployment is meant to quell growing criticism in communities in the south; also, the rocket volleys as far north as Ashdod and Gan Yavneh last week blurred the line between the south and the center of the country.

Iron Dome's deployment can have two immediate results, each of them negative in one sense or another. The first would stem from the system's failure. Hamas or Islamic Jihad would fire rockets at a major civilian target. Iron Dome would identify the approaching threat and fire an interceptor missile but miss the incoming rocket, or the missile would explode without destroying it. That would be an embarrassing failure that would harm Israeli deterrence and subject to ridicule the pretensions that politicians, defense contractors and to a lesser extent army officers have regarding the system.

The second possible problematic outcome involves success in intercepting the Qassam or Grad rocket. This would prove that the Israeli government and defense establishment learned from their mistakes from 2003 to 2006. The suggestion was made at the time that such systems be developed to protect northern communities from Hezbollah and southern communities from Hamas, but this was rejected out of hand.

Success will prove the wisdom of adding - even if belatedly - two lower layers of protection, Iron Dome and just above it Magic Wand, to the upper layers of protection from the Arrow 3 and Patriot systems. But such success would just whet appetites. Unprotected communities would envy protected ones, and the people launching the rockets would probably choose new targets - exposed communities. Or they would use direct short-range weaponry (mortars and anti-tank weapons ) on kibbutzim and moshavim on the border.

Iron Dome's success won't spare Israel dilemmas over which expensive equipment to develop involving a number of systems; it won't even spare Israel demands to engage in offensive operations. It's good that anti-rocket defense will save lives, but it's no substitute for far-sighted policies, whose range is even longer than the missiles'.