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US Secretary of Defense Chuck Hagel says the American military is ready to strike Syria at any time. The Pentagon has positioned destroyers and warplanes to carry out whatever attack option President Obama chooses, Mr. Hagel tells BBC News.

All that鈥檚 left is for Mr. Obama to give the order. As yet, the US chief executive has not decided what sort of armed response the US will make to Syria鈥檚 use of chemical weapons against its own people, say US officials.

鈥淗e has seen [the options]. We are prepared. We are ready to go,鈥� Hagel says.

Given that, what would a US attack look like?

First, it will probably start at night. US night-fighting capability is unsurpassed, and night attacks reduce the risk of civilian casualties, given that any civilian workers at Syrian military installations are likely to be home in bed. This could occur within days, perhaps as early as Thursday.

Second, the weapon of choice will almost certainly be precision-guided munitions. The US Navy has four destroyers within range of Syrian targets. Each Arleigh Burke-class destroyer has 90 vertical launchers for Tomahawk land attack cruise missiles and defensive missiles, according to a Syria attack plan produced by Christopher Harmer, senior naval analyst at the Institute for the Study of War. Depending on the mix of munitions loaded in these launchers, four ships should easily be able to hit Syrian targets with 180 Tomahawks.

US cruise missiles have a 1,000-mile range, meaning they can be launched hundreds of miles at sea. If they operate as intended, their accuracy carries them to within a few meters of their intended targets.

Carrier-launched F-18E aircraft and F-15E land-based warplanes could augment a cruise-missile strike with their own standoff weapons.

A mix of 24 cruise missiles, 24 missiles from F-18s, and 24 missiles from F-15s would degrade Syrian Air Force facilities to the point where the regime of Bashar al-Assad would find it impossible to carry out resupply operations or attack rebel forces from the air, according to the Institute for the Study of War.

鈥淭he entire strike scenario can be conducted without any US aircraft entering Syrian airspace. All weapon launches can take place from international airspace over the Mediterranean, or over Turkish, Israeli, Jordanian, or Saudi airspace,鈥� writes ISW鈥檚 Mr. Harmer.

Third, the most important question regarding the attack plan may not be how it starts, but how it ends. What is its purpose? To deter the Syrian regime from further chemical use? To destroy the regime鈥檚 chemical stocks? To degrade the Syrian Air Force? To eliminate the Syrian Air Force entirely?

These different options would require different military actions after the initial shock of a cruise-missile attack.

Syria has five main chemical weapons bases, for instance. But such weapons are easy to hide elsewhere, and a strike against a chemical base risks inadvertent release of poison gas.

鈥淓ven the most successful cruise missile strikes would not destroy Syria鈥檚 [chemical weapon] holdings," writes Anthony Cordesman, a military and foreign-policy expert at the Center for Strategic and International Studies, in an analysis of Syrian attack options.

鈥淭here is no credible chance the US can locate or destroy Syria鈥檚 entire [chemical weapon] holding without a massive air campaign and some kind of presence on the ground. Even if the Assad regime has not done the obvious, and used the last few months to covertly disperse a large portion of its weapons, cruise missiles simply don鈥檛 have that kind of destructive power,鈥澛爃e adds.

Syria has about 100 usable fixed-wing military aircraft, and 27 primary air bases. Rendering these inoperable is an achievable goal, but again, that is likely to require follow-up strikes and sorties by manned aircraft over Syria, endangering US pilots.

Syria has a more robust air defense capability than did Libya under Muammar Qaddafi, or Iraq under Saddam Hussein. Its fixed sites could be taken out by precision US weapons, but mobile ground radars and rockets would pose a continuing danger to American air power.

Given these risks, the tactical action of launching a strike makes no sense in the absence of a strategic goal, says the author of the ISW attack plan.

鈥淚f the US is going to become militarily involved in Syria 鈥� and there are good arguments for doing so, as well as important cautions 鈥� then President Obama absolutely must explain clearly and cogently what it is he is trying to achieve,鈥� writes Harmer in a new ISW commentary.