At 10 a.m. Tuesday, the wailing of sirens will be heard throughout Israel - except in the southern communities near Gaza under intermittent Palestinian bombardment. Israel has begun a week-long civil defense drill. On Sunday the cabinet was briefed by Deputy Defense Minister Matan Vilna'i, who is commanding the nationwide exercises. Everyone from kindergarten children to senior civil servants will practice the emergency measures to be taken in the event of an actual attack or catastrophic earthquake. These national home front exercises are intended to help authorities evaluate how prepared the country is to face the threat, for instance, of missiles - conventional, biological, chemical or nuclear - smashing into our population centers. Are rescue services geared up to save casualties from dozens of simultaneously collapsed buildings? Are hospitals capable of treating mega-casualties? Prime Minister Ehud Olmert and Defense Minister Ehud Barak, along with officials of the Home Front Command, National Emergency Authority and emergency services, will spend the better part of the week simulating potential disaster scenarios. A drill of this magnitude has never been undertaken. Yet we don't need a drill to realize that the Home Front Command Web site (www.oref.org.il), currently limited to Hebrew and English, should do a better job of communicating with Russian, Arabic and Amharic speakers; and that authorities need to clarify their zigzagging policies regarding gas masks. For our part, we citizens must take these exercises seriously - specifically, locating the closest shelter or secure room when the siren sounds. While this is only a drill, it comes as tensions are high on the northern front over concern that Hizbullah may attempt a massive terrorist operation, ostensibly in retaliation for the mysterious liquidation in Damascus of its top commander, Imad Mughniyeh. Jerusalem has signaled Bashar Assad's regime that it might hold Syria accountable if Hizbullah strikes. That, in turn, probably led Damascus to leak a story to the London-based Al Quds al-Arabi about Syrian reservists mobilizing "in anticipation" of an Israeli attack. Conflicting reports remain over whether and to what extent the Syrian army is in fact massing troops. Tensions are further exacerbated by reports of authorities continuing to worry that enemy forces may try to down - or hijack - an Israeli airliner. Extraordinary preventative measures are under way. But has Israel's deterrence so deteriorated that terrorists and/or their state benefactors - Syria and Iran - would even consider such a heinous assault? Are they unmindful of the devastating consequences? Equally troubling are unconfirmed reports that Russia has now joined Iran in providing personnel for Syrian monitoring stations which also feed data to Hizbullah. Meanwhile, Israel remains alert (in the north and south) against the threat that its soldiers may be kidnapped, or that Hizbullah may send bomb-laden drones to infiltrate Israeli airspace. As if the situation were not sufficiently fraught, authorities are surely also trying to make sense of the recording by al-Qaida leader Ayman al-Zawahiri released last week. Entitled "Rush to Support Our People in Gaza," Zawahiri urges Islamists to strike at Jewish and American interests everywhere. But Noah Shachtman, national security blogger for Wired.com, has called attention to an intriguing aside in Zawahiri's message, namely, what he says about Iran: "The dispute between America and Iran is... over areas of influence... the situation will be in the interest of the Mujahideen if the war saps both of them. If, however, one of them emerges victorious, its influence will intensify and fierce battles will begin between it and the Mujahideen..." Zawahiri thus inadvertently reminds us - at a time when it feels as if we're surrounded on all sides - that Sunni-Shi'ite and Arab-Persian cleavages have not been buried. Islamists may cooperate on the tactical level, but the enemy is neither monolithic nor unified. As Israelis prepare for a disaster we pray will never come, today's sirens cannot help but underscore the threats that surround the Jewish state. Let them, equally, ring out our readiness to defeat any foe.