Syria war explained: Here's what you need to know

World leaders continue to weigh up military action in response to a suspected poison gas attack in Syria over the weekend.

Top officials in the U.K. are scheduled to convene on Thursday after British Prime Minister Theresa May called for talks to work out the country's response to the situation in Syria. The scheduled meeting follows reports Westminster has ordered submarines to move within missile range of the war-torn nation in readiness for potential strikes.

The prospect of British military action comes less than 24 hours after President Donald Trump taunted Russia with the threat of imminent missile strikes in Syria. The Kremlin immediately responded to the U.S. president, saying missiles "should fly toward terrorists," and not its own legal government bases in the Middle Eastern country.

CNBC takes a look at how a peaceful uprising in Syria seven years ago has turned into a full-scale civil war.

Trump to Russia: 'Get ready'

In an incendiary tweet on Wednesday, Trump said Russia should "get ready" for missiles to be fired into Syria.

The move comes on the back of a recent suspected chemical attack on Douma, the last rebel-held town in the Eastern Ghouta region of Syria. Opposition activists, rescue workers and medics working in the country said that more than 40 people were killed after bombs filled with toxic chemicals were dropped by Syrian government forces.

President Bashar Assad, who receives military backing from Moscow, has denied being responsible for any chemical attack.

Russia's ambassador to Lebanon said late Tuesday that any U.S. missiles fired at Syria would not only be shot down but the launch sites of those missiles would also be targeted. The prospect of a retaliatory response from Russia on U.S. "launch platforms" could subsequently prompt a dramatic escalation in the Syrian war.

Responding to the Russian ambassador's comments, General Richard Barrons, who led the U.K.'s Joint Forces Command from 2013 to 2016, told BBC's Radio 4 "Today" program on Wednesday: "I hope the ambassador has chosen his words very carefully… He's saying they are going to try and sink ships, sink submarines and shoot aircraft out of the sky — that's war."

Why is there a war?

The war has changed dramatically over the last seven years, rapidly morphing into much more than a battle between those who are simply for and against President Assad.

Several countries and groups are involved — each with its own agenda while collectively making the situation extremely complex.

Key supporters of Syria's administration include Russia and Iran, while the U.S., Turkey and Saudi Arabia all back government rebels.

Last month, Amnesty International urged world leaders to double down on their efforts to urgently end the suffering of millions of Syrian citizens and bring an end to the "bloody assault" that has been raging for more than seven years.

"The international community's catastrophic failure to take concrete action to protect the people of Syria has allowed parties to the conflict, most notably the Syrian government, to commit war crimes and crimes against humanity with complete impunity, often with assistance of outside powers, particularly Russia," Lynn Maalouf, Middle East research director at Amnesty International, said in a statement on the group's website.

"Every year, we think it is just not possible for parties to the conflict to inflict more suffering on civilians, and yet, every year, they prove us wrong," she added.

How have Syrian people been impacted?

By March, the Syrian Observatory for Human Rights had documented the deaths of over 350,000 people — including 106,000 civilians.

The figure, gathered by the U.K.-based monitoring group's network of sources in the war-torn country, does not include 56,900 people who it said were missing and presumed dead.

The group also projected about 100,000 deaths had not been documented.

The United Nations High Commissioner for Refugees (UNHCR) estimates that around 11 million Syrians have fled their homes since the outbreak of the civil war in March 2011, with the majority seeking refuge in neighboring countries or within Syria itself.

The pre-war population of Syria was roughly 22 million, meaning so far around half of the country has been displaced.

What is the UN doing about the alleged chemical attack?

Members of the United Nations Security Council (UNSC) met Tuesday evening in an attempt to agree on a concerted international response to the alleged use of chemical weapons.

However, rival draft resolutions by Washington and Moscow to try and set up a new expert body to properly assess the attack both failed to pass.

Russia first vetoed a U.S.-drafted text to create a panel that would have the authority to assign blame for any chemical attacks in the Middle Eastern country. The resolution received 12 votes in favor but Russia and Bolivia both voted against it — while China abstained.

The U.S.-drafted resolution required nine votes in favor and no vetoes by any of the UNSC's permanent members. These include Russia, China, the U.K., France and the U.S.

It was the 12th time Russia had used its veto powers to block action against the Assad government.

Later, Russia failed to pass with its own bid to create a new inquiry after it only received six votes in favor.

What happens next?

The U.S. is widely expected to respond to the recent alleged chemical attack with missile strikes. Last year, the Trump administration authorized the launch of 59 Tomahawk missiles on a government airfield in Syria after it said the targeted airfield had launched a chemical attack.

Russia has vowed to shoot down any missiles that enter Syrian airspace —and even suggested targeting missile launch sites.

"The most likely Russian response to an air campaign that targets Assad's forces, but leaves Russian troops unharmed, is the use of electronic warfare to limit the potential damage on the military infrastructure in the country," Ayham Kamel, head of Eurasia Group's Middle East and North Africa practice, said in a research note Monday.

He argued that such a move would represent "the most benign escalation scenario where Russia grudgingly accepts the new reality."

"At the same time, airstrikes could be a prelude to a cycle of escalation…," Kamel added. "The most significant risk that could emerge of the current dynamics is a U.S. decision to expand the scope of military operations in a manner that would present more significant risks to Russian forces in Syria."

Source: Bloomberg Pro Terminal

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