FILE - EDS NOTE: GRAPHIC CONTENT - This Sunday, April. 8, 2018 file image released by the Syrian Civil Defense White Helmets, shows a rescue worker carrying a child following an alleged chemical weapons attack in the rebel-held town of Douma, near Damascus, Syria. With the Middle East on edge and many fearing inadvertent triggering of regional war, it is easy to forget that two weeks ago Trump shocked advisers in declaring an intention to withdraw troops from Syria. Now, apparently angered by a suspected chemical attack, Trump is threatening imminent military strikes against the Syrian government forces he blames and rattling a saber at Syria’s patron Russia. (Syrian Civil Defense White Helmets via AP, File)

BEIRUT - With the Middle East on edge and many fearing inadvertent triggering of a regional war, it's easy to forget that not so long ago President Donald Trump shocked advisers by declaring his intention to withdraw U.S. troops from Syria and "let others take care of it now."

Now, Trump is threatening imminent military strikes against the Syrian government forces he blames for a suspected chemical gas attack and rattling a saber at Syria's patron, Russia. Missiles "will be coming, nice and new and 'smart,'" Trump warned in a tweet.

The suspected chemical weapons attack on a rebel-held town near Damascus killed more than 40 people over the weekend, many of them said to have suffocated in underground shelters from a chlorine-like substance. The attack was quickly and widely blamed on President Bashar Assad's forces, provoking calls from Western nations for swift retribution. But Assad and his backers in Russia have denied it.

Russia has warned the United States that a strike on Syria could trigger a potential major showdown between the two former Cold War adversaries — a prospect not considered in decades. A senior Russian lawmaker said Wednesday that Russia would engage its warships in the Mediterranean Sea to protect Russian assets in Syria from a U.S. strike.

In a landscape that includes Iran and Israel seemingly straining for a fight, and NATO member Turkey occupying part of the country, the possible escalation scenarios are clear.

Now a scramble is on to prepare for whatever might come next. Here's a look at how the sides reached this point:

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THE SUSPECTED ATTACK

Trump says he has little doubt that Syrian government forces were to blame for the suspected gas attack in the town of Douma, but neither he nor other U.S. administration officials have produced hard evidence. U.S. officials have suggested such evidence is lacking, or at least not yet in hand.

This is in contrast to an incident a year ago when U.S. intelligence agencies had video and other evidence of an actual attack involving sarin nerve gas. Trump responded by launching Navy cruise missiles at a Syrian airfield.

A U.S. official said evidence collected so far by Western governments bears the same hallmarks as previous chemical attacks by Assad's government, including the position of the bodies and the lack of outwardly visible wounds that would result from a conventional attack.

There is also no evidence to suggest Assad's government was being framed for the latest suspected attack, said the official, a reference to past accusations by the Syrian government that the rebels might themselves release gas so that Assad would be blamed and they would garner sympathy. The official spoke on condition of anonymity because he wasn't authorized to publicly discuss the investigation or plans for any military response.

Russia and the Syrian government strongly deny the allegations or that any chemical weapons attack took place.

Paramedics said many of the victims in Douma were brought to medical centres complaining of difficulty breathing, foaming at the mouth and burning sensations in the eyes. The U.N. health agency said reports from its partners indicate some 500 patients showed signs of exposure to toxic chemicals following the shelling of Douma.

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WHY NOW?

Trump, like his predecessor, President Barack Obama, has stated that a chemical weapons attack is a red line. Last year's retaliatory strike was a sign he is serious.

With the suspected gas attack in Douma, he feels obliged to keep his word and strike again, presumably with a more forceful response than last year, which apparently did not deter future chemical attacks. The question is how forceful a strike would be needed to achieve results while avoiding triggering a Russian response.

A narrow, limited attack is unlikely to deter repeated chemical weapons use, while a broader one risks triggering dangerous consequences.

"Whatever strikes he might order will need to hit Assad hard, and beyond symbolic targets of the regime's authority in Damascus," said Nicholas A. Heras, a Middle East security fellow at the Washington-based Center for a New American Security.

For the more cynical observer, part of the equation may be that both Trump and Israeli Prime Minister Benjamin Netanyahu could use a diversion from domestic legal troubles. Trump is embroiled in the investigation by special counsel Robert Mueller and is fuming at federal agents digging into his personal affairs. Netanyahu faces an array of corruption scandals, with police recommending his indictment in two cases.

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SCENARIOS AND CONSEQUENCES

On the less dramatic end of the scale would be a strike similar to last year's U.S. missile attack that targeted an air base in central Syria, possibly on a larger scope, but largely amounting to a slap on the wrist.

"The Trump team is balancing Mr. Trump's desire to show (Assad) who is the boss, with the reality that both Russia and Iran will need to show their own resolve against the United States, or face the risk of being viewed as paper tigers," Heras said.

Russia and Iran both maintain troops at air bases across government-controlled areas of Syria. Russia has warned that any strike that hurts Russian soldiers would bring about swift retribution. Iran also has numerous options to respond to any American punitive attack in Syria, hurting the U.S. or its allies in the region, including Saudi Arabia.

Already Iran is threatening retaliation for an attack on a Syrian air base this week that killed seven Iranians — and that it, Syria and Russia have blamed on Israel.

At the other end of the spectrum is a nightmare scenario that brings the U.S. and Russia into direct confrontation and pulls in Iran and Israel — with Iran using the Shiite militia Hezbollah as a proxy to attack Israel with long-range rockets, with calamitous consequences for neighbouring Lebanon, where Hezbollah is based.

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POSSIBLE TARGETS

Possible targets range from limited strikes on sites and factories related to chemical weapons production, as well as security units or command centres that carried out last week's attack, to wider-scale attacks that seek to take out Assad's air defences and air force.

There are 25 air bases in Syria, including 21 under government control. The biggest and most active is Hemeimeem, where Russia has dozens of aircraft and a sophisticated air-defence system. Others include Shayrat, the base the U.S. hit with 59 cruise missiles in April 2017 in retaliation for the suspected sarin gas attack, and T4, the base targeted by the suspected Israeli airstrike this week.

According to opposition activists, another possible target could be al-Dumeir air base near Damascus, where the helicopters that carried out Saturday's alleged chemical weapons attack took off from.

Opposition activists have said that Syrian forces have been on high alert at all military bases, from Sweida province in the south, to Aleppo province in the north, to the Mediterranean coast in the west to Deir el-Zour in the east, along the Iraqi border.

The Britain-based Syrian Observatory for Human Rights said Syrian forces have evacuated several bases, including Thaala in the south and T4 in central Syria.

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Associated Press writers Josh Lederman in Washington and Sarah El Deeb in Beirut contributed to this report.

News from © The Associated Press, 2018