The head of Israeli military intelligence warned his political superiors that the next major war Israel encountered would likely result in much higher Israeli casualties, especially to civilians. There is also growing concern about Russia selling advanced anti-aircraft systems to Syria, which could pass them on to Iran.
All this could be traced back to preparations Iran has been making for over a decade. Using their oil wealth, and weapons smuggling network, Iran has armed Syria, Hezbollah (the Shia militia in southern Lebanon) and Hamas (the Palestinian terrorist group that runs Gaza) with over 50,000 rockets, plus numerous other weapons. Most of the rockets are short range (about 10 kilometers), but several thousand have a much longer reach, and can hit targets throughout Israel. The Iranian master plan is for Hezbollah, Hamas, Syria and Iran to simultaneously fire as many missiles and rockets into Israel as they can. Even if there are no ground forces to follow up such an attack, the casualties (civilian and military) in Israel would be seen as a great Islamic victory, and would demoralize the Israelis. While Israeli defensive moves could do great damage to Syria, Lebanon and Gaza, Iran considers it a reasonable plan. Hezbollah, Syria and Hamas are not so sure, but Iran already has all the rockets and missiles in place.
Israeli military planners have seen this coming. Over the last three years, Israel has been revising its civil defense plans, and how to deal with the growing arsenal of rockets and ballistic missiles aimed at it. The latest change is the announcement that the military is dispersing its stocks of supplies, equipment and spare parts to a larger number of (better protected) locations.
The basic defense plan assumes a future war with Syria, and gives the local officials an idea of what to expect. Currently, the Israelis estimate that there would be as many as 3,300 Israeli casualties (including up to 200 dead) if Syria tried to use its long range missiles against Israel. If the Syrians used chemical warheads, Israeli casualties could be as high as 16,000. Over 200,000 Israelis would be left homeless, and it's believed about a 100,000 would seek to leave the country.
Israel now assumes that Iran would also fire some of its ballistic missiles as well, armed with conventional warheads. But the big danger is Syria, which is a client state of Iran. Syria has underground storage and launch facilities for its arsenal of over a thousand SCUD missiles. Armed with half ton high explosive and cluster bomb warheads, the missiles have ranges of 500-700 kilometers. Syria also has some 90 older Russian Frog-7 missiles (70 kilometer range, half ton warhead) and 210 more modern Russian SS-21 missiles (120 kilometer range, half ton warhead) operating with mobile launchers. There are also 60 mobile SCUD launchers. The Syrians have a large network of camouflaged launching sites for the mobile launchers. Iran and North Korea have helped Syria build underground SCUD manufacturing and maintenance facilities. The Syrian missiles are meant to hit Israeli airfields, missile launching sites and nuclear weapons sites, as well as population centers. Syria hopes to do enough damage with a missile strike to cripple Israeli combat capability.
Israel has long been aware of the Syrian capabilities and any war with Syria would probably result in some interesting attacks on the Syrian missile network. The SCUD is a liquid fuel missile and takes half an hour or more to fuel and ready for launch. So underground facilities are a major defensive measure against an alert and astute opponent like Israel.
But Syria has been adding a lot of solid fuel ballistic missiles to its inventory, and recently transferred some of these to Hezbollah, in Lebanon. Hezbollah and Syria would likely coordinate an attack on Israel. Hamas, in Gaza, is a semi-client of Iran, and might be persuaded to join in as well.
No unclassified government planning documents have discussed what Israel would do in response to such an attack, but in the past, Israel has threatened to use nukes against anyone who fired chemical weapons at Israel (which does not have any chemical weapons). But current plans appear to try and keep it non-nuclear for as long as possible.