Despite president Barack Obamas pledge that Syrias use of chemical weapons is a game changer for the United States, he is unlikely to turn to military options quickly and would want allies joining him in any intervention.
Possible military choices range from limited one-off missile strikes from ships - one of the less complicated scenarios - to bolder operations like carving out no-fly safe zones.
One of the most politically unpalatable possibilities envisions sending tens of thousands of US forces to help secure Syrian chemical weapons.
Obama has so far opposed limited steps, like arming anti-government rebels, but pressure to deepen US involvement in Syrias civil war has grown since last months White House announcement that president Bashar al Assad likely used chemical weapons.
After fighting wars in Afghanistan and Iraq, the Pentagon is wary of US involvement in Syria. The presidents top uniformed military adviser, general Martin Dempsey, said in March he could not see a US military option with an understandable outcome there.
Theres a lot of analysis to be done before reaching any major decisions that would push US policy more in the direction of military options, a senior US official told Reuters.
That caution is understandable, given the experience of Iraq where the United States went to war based on bad intelligence about weapons of mass destruction. The Pentagon has made repeated warnings of the enormous risks and limitations of using American military might in Syrias civil war.
One form of military intervention that could to some extent limit US and allied involvement in Syrias war would be one-off strikes on pro-Assad forces or infrastructure tied to chemical weapons use. Given Syrias air defenses, planners may choose to fire missiles from ships at sea.
The most proportional response (to limited chemical weapons use) would be a strike on the units responsible, whether artillery or airfields, said Jeffrey White, a former senior official at the Pentagons Defense Intelligence Agency and a Middle East expert who is now a defense fellow at the Washington Institute For Near East Policy.
It would demonstrate to Assad that there is a cost to using these weapons - the problem so far is that theres been no cost to the regime from their actions.
It is not clear how the Syrian government would respond and if it would try to retaliate militarily against the US forces in the region. US military involvement would also upset Russia which has a naval facility on Syrias Mediterranean coast.
Another option that the Pentagon has examined involves the creation, ostensibly in support of Turkey and Jordan, of humanitarian safe areas that would also be no-fly zones off limits to the Syrian air force - an option favored by lawmakers including Senator John McCain of Arizona.
This would involve taking down Syrian air defenses and destroying Syrian artillery from a certain distance beyond those zones, to protect them from incoming fire.
Advocates, including in Congress, say a safe zone inside Syria along the Turkish border, for example, would give needed space for rebels and allow the West to increase support for those anti-Assad forces it can vet.
Still, as officials, including defense secretary Chuck Hagel, have warned, once established, a safe zone would tie the US more closely to Syrias messy conflict. Assad would almost certainly react.
Once you set up a military no-fly zone or safe zone, youre on a slippery slope, mission creep and before you know it, you have boots on the ground, said Bruce Riedel, a former CIA analyst and Middle East expert at the Brookings Institution.
Or you end up like Libya where you dont really have a control mechanism for the end-game, should you end up with chaos.
The US military has also completed planning for going into Syria and securing its chemical weapons under different scenarios, including one in which Assad falls from power and his forces disintegrate, leaving weapons sites vulnerable to pillaging.
The US fears anti-Assad Islamist rebels affiliated to al Qaeda could grab the chemical weapons but a US intervention into Syria to get the arms would require tens of thousands of American troops.
Asked if he was confident the US military could secure Syrias chemical weapons stock, Dempsey told Congress: Not as I sit here today simply because they have been moving it and the number of sites is quite numerous.
Obama said last month he would seek to mobilise the international community around Syria, as he attempts to determine whether pro-Assad forces used chemical weapons.
British and French officials have long made it clear their countries might be willing to join in any US-led action under the right circumstances.
But Hagel warned that no international or regional consensus on supporting armed intervention now exists. Once a fervent advocate of foreign intervention in Syria, Turkey has grown frustrated with the fractured opposition to Assad and with international disunity.
NATO chief Anders Fogh Rasmussen has ruled out western military intervention and US Admiral James Stavridis, NATOs supreme allied commander, cautioned in March that the alliance would need agreement in the region and among NATO members as well as a UN Security Council resolution - something that looks unlikely given probable opposition from Russia and China.
It seems increasingly clear that the Obama administration is feeling pressure to act, said Mona Yacoubian, a former State Department official and now a Syria expert at the Stimson Center in Washington.
But they will likely seek two things: conclusive evidence and multilateral support/participation in whatever action (they) choose, which I think would be limited, targeted air strike.
