More than 465,000 Syrians have been killed in the fighting, over a million injured, and over 12 million - half the country's prewar population - have been displaced.

Here is how and why the conflict started:

What caused the uprising?

While lack of freedoms and economic woes drove resentment of the Syrian government, the harsh crackdown on protesters inflamed public anger.

Arab Spring: In 2011, successful uprisings - that became known as the Arab Spring - toppled Tunisia's and Egypt's presidents. This gave hope to Syrian pro-democracy activists.

That March, peaceful protests erupted in Syria as well, after 15 boys were detained and tortured for writing graffiti in support of the Arab Spring. One of the boys, a 13-year-old, was killed after having been brutally tortured.

The Syrian government, led by President Bashar al-Assad, responded to the protests by killing hundreds of demonstrators and imprisoning many more.

In July 2011, defectors from the military announced the formation of the Free Syrian Army, a rebel group aiming to overthrow the government, and Syria began to slide into war.

While the protests in 2011 were mostly non-sectarian, the armed conflict surfaced starker sectarian divisions. Most Syrians are Sunni Muslims, but Syria's security establishment has long been dominated by members of the Alawi sect, of which Assad is a member.

In 1982, Bashar's father ordered a military crackdown on the Muslim Brotherhood in Hama, killing tens of thousands of people and flattening much of the city.

Even global warming is said to have played a role in sparking the 2011 uprising. Severe drought plagued Syria from 2007-10, causing as many as 1.5 million people to migrate from the countryside into cities, exacerbating poverty and social unrest.

International involvement

Foreign backing and open intervention have played a large role in Syria's war. Russia entered the conflict in 2015 and has been the Assad government's main ally since then.

Regional actors: The governments of majority-Shia Iran and Iraq, and Lebanon-based Hezbollah, have supported Assad, while Sunni-majority countries, including Turkey, Qatar, and Saudi Arabia supported anti-Assad rebels.

Since 2016, Turkish troops have launched several operations against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) near its borders, as well as against Kurdish groups armed by the United States.

Anti-ISIL coalition: The US has armed anti-Assad rebel groups and led an international coalition bombing ISIL targets since 2014.

Israel carried out air raids inside Syria, reportedly targeting Hezbollah and pro-government fighters and facilities.

The first time Syrian air defences shot down an Israeli warplane was in February 2018.

US and Russia

The US has repeatedly stated its opposition to the Assad government backed by Russia but has not involved itself as deeply.

Chemical red line: Former US President Barack Obama had warned that the use of chemical weapons in Syria was a "red line" that would prompt military intervention.

In April 2017, the US carried its first direct military action against Assad's forces, launching 59 Tomahawk cruise missiles at a Syrian air force base from which US officials believe a chemical attack on Khan Sheikhoun had been launched.

One year later, on April 14 , despite Russian warnings, the US launched an attack together with France and the UK, at "chemical weapon sites".

, despite Russian warnings, the US launched an attack together with France and the UK, at "chemical weapon sites". CIA training: In 2013, the CIA began a covert programme to arm, fund and train rebel groups opposing Assad, but the programme was later shut down after it was revealed that the CIA had spent $500m but only trained 60 fighters.

Russia's campaign: In September 2015, Russia launched a bombing campaign against what it referred to as "terrorist groups" in Syria, which included ISIL as well as anti-Assad rebel groups backed by the USA. Russia has also deployed military advisers to shore up Assad's defences.

At the UN Security Council, Russia and China have repeatedly vetoed Western-backed resolutions on Syria.

Peace talks

Peace negotiations have been ongoing between the Syrian government and the opposition in order to achieve a military ceasefire and political transition in Syria, but the main sticking point has been the fate of Assad.

Geneva: The first round of UN-facilitated talks between the Syrian government and opposition delegates took place in Geneva, Switzerland in June 2012.

The latest round of talks in December 2017 failed amid a tit-for-tat between the Syrian government and opposition delegates over statements about the future role of Assad in a transitional government.

In 2014 Staffan de Mistura replaced Kofi Annan as the UN special envoy for Syria.

Astana: In May 2017, Russia, Iran and Turkey called for the setup of four de-escalation zones in Syria, over which Syrian and Russian fighter jets were not expected to fly.

After denouncing plans to partition Syria in March 2018, a follow-up trilateral summit was held in Turkey to discuss the way forward.

Sochi: In January 2018, Russia sponsored talks over the future of Syria in the Black Sea city of Sochi, but the opposition bloc boycotted the conference, claiming it was an attempt to undercut the UN effort to broker a deal.

Since the conflict began, as a Syrian rebellion against the Assad government, many new rebel groups have joined the fighting in Syria and have frequently fought one another.

The Free Syrian Army (FSA) is a loose conglomeration of armed brigades formed in 2011 by defectors from the Syrian army and civilians backed by the United States, Turkey, and several Gulf countries.

In December 2016, the Syrian army scored its biggest victory against the rebels when it recaptured the strategic city of Aleppo. Since then, the FSA has controlled limited areas in northwestern Syria.

In 2018, Syrian opposition fighters evacuated from the last rebel stronghold near Damascus. However, backed by Turkey, the FSA took control Afrin, near the Turkey-Syria border, from Kurdish rebel fighters seeking self-rule.

ISIL emerged in northern and eastern Syria in 2013 after overrunning large portions of Iraq. The group quickly gained international notoriety for its brutal executions and its energetic use of social media to recruit fighters from around the world.

Other groups fighting in Syria include Jabhat Fateh al-Sham, Iran-backed Hezbollah, and the Syrian Democratic Forces (SDF) dominated by the Kurdish People's Protection Units (YPG).

Fighting in Syria continues on several fronts:

Idlib: In February 2018, shelling by Russian and Syrian forces have intensified on Idlib, especially since fighters from the Hay'et Tahrir al-Sham group shot down a Russian warplane.

In April, Russia brokered a deal to evacuate opposition fighters from Eastern Ghouta in the south to Idlib in the north, Idlib being one of the few strongholds controlled by opposition fighters.

The province is strategically important for the Syrian government and Russia for its proximity to the Russian-operated Syrian Khmeimim airbase.

Homs: In April, an airbase and other Syrian government facilities in Homs became again the target of Israeli and US-led air strikes in which UK and French forces also participated.

The Syrian army recaptured the city of Homs in 2014, but fighting continues with rebels in the suburbs between Homs and Hama.

Afrin: Turkey and the Free Syrian Army (FSA) began in January 2018 a military operation against US-backed fighters in northwestern Syria, and announced the capture of Afrin's city centre in March.

US troops are stationed in nearby Manbij, prompting fears of a US-Turkey confrontation.

Who is controling what in Syria? https://t.co/MlPM1aJf4Q pic.twitter.com/qz2dY2qvT0 — Al Jazeera English (@AJEnglish) March 3, 2018

Now having gone on longer than World War II, the war in Syria is causing profound effects beyond the country's borders, with many Syrians having left their homes to seek safety elsewhere in Syria or beyond.