The US-led coalition fighting the Islamic State group has avoided involvement in Syria's civil war, but on Thursday it said it killed more than 100 pro-regime fighters in the country.
The international coalition was set up in 2014 to drive IS from territories it controlled in Iraq and Syria.
Washington has deployed 2,000 soldiers in Syria, mainly special forces, who support the Syrian Democratic Forces (SDF), an Arab-Kurdish coalition fighting the IS.
- First air strikes -
In September 2014, the US and Arab allies launch air strikes on the IS in Syria, opening a new front in the fight against the jihadist group, already targeted by raids in Iraq.
- Support for Kurds -
In October 2014 the US State Department reveals that American officials have met for the first time the Kurdish Democratic Union Party (PYD).
The PYD is the political branch of the People's Protection Units (YPG), which are Syria's main Kurdish militia.
In January 2015 the Kurdish militia drives IS out of the Kurdish border town of Kobane, after more than four months of fighting backed by coalition air strikes.
The militia becomes the spearhead on Syrian soil of the coalition's fight against the jihadists.
- Errors -
In September 2016, a string of coalition air strikes hit a Syrian army base, killing as many as 90 regime soldiers in the eastern region of Deir Ezzor.
Washington says it believed it was targeting an IS position there, but Damascus says the strike was "intentional".
The Syrian Observatory for Human Rights monitor has reported several mistakes by the coalition, which says it takes every precaution to avoid civilian casualties.
- Pro-regime forces bombed -
In April 2017 Syrian warplanes strike the rebel-held town of Khan Sheikhun in Idlib with chemical agent sarin, killing more than 80.
On the night of April 6-7, the US military hits back by firing 59 Tomahawk missiles from warships in the Mediterranean at the central Shayrat airbase, which it says was the launchpad for the chemical attack.
In May, coalition aircraft strike a pro-regime convoy as it approaches a training garrison near the Al-Tanf border crossing to Jordan. The US had chosen this town to train local Syrian forces to fight IS.
In June a US warplane shoots down a Syrian jet for the first time after it bombs SDF fighters. Syrian ally Russia denounces the "aggression".
- Arms for Kurds -
In May 2017 the White House gives the Pentagon the green light to deliver weapons to the YPG.
The decision provokes anger in Turkey, which says the YPG is linked to its own domestic Kurdish separatist group the PKK, or Kurdish Workers Party.
It is the first time a US administration officially supplies arms to the Kurds, with the Americans having up until then saying they would only arm the Kurds' Arab allies.
In October 2017 the Kurdish-Arab coalition drives IS out of its Syrian stronghold Raqa after months of fighting.
- Pro-regime forces hit -
On February 8, 2018, the coalition says it killed at least 100 pro-regime fighters to fend off an attack on its allies in eastern Syria.
Syria brands the strikes, in Deir Ezzor province, a "war crime".