U.S. Army Patriot anti-aircraft missile units will arrive in Israel this week for joint exercises with Israeli anti-aircraft units. The thousand American troops will actually practice shooting down SCUD missiles. The Israeli Arrow anti-missile system, developed jointly with a lot of American money and some U.S. technology, has passed several tests, but has not been used in combat yet. Each Arrow missile costs three million dollars. Arrow can hit incoming missiles up to a hundreds kilometers away, while the Patriot intercepts at a much shorter range (under ten kilometers.) The U.S./Israeli exercise will test using Patriots and Arrow together to provide a two layer defense. Israel has Patriot missiles and has apparently already worked out the details of this kind of system. The U.S. has the option to buy Arrow systems, which may happen if Arrow proves very successful against Iraqi SCUDs. Israel has said it will strike back if Iraq fires SCUDs at Israel. But Israeli politicians will apparently hold their fire if the Iraqi SCUDs are intercepted by Arrow and Patriot missiles, or if only one or two get through and cause few casualties. If an Iraqi SCUD carrying chemical or biological weapons lands in Israel, there probably would be retaliation. The United States does not want Israel attacking Iraq, as this would cause political problems with other Arab nations.