The US-backed Syrian Democratic Forces alliance of Kurdish and Arab fighters that entered the Islamic State (IS) militant group's bastion Raqa on Wednesday was formed just 18 months ago.
It has since emerged as a key fighting force against IS in northern Syria, and the main ground partner of the US-led coalition battling IS.
The SDF has scored a series of victories in the past 18 months, including the recapture of the strategic northern city of Manbij, and has now encircled Raqa city and entered it from the east.
But the dominant role of Kurdish forces in the alliance has raised concerns with Turkey, whose military has hit SDF positions.
The SDF was formed in October 2015 as Kurds, Arab Muslims and Christians joined forces to battle IS in northern Syria.
Syria's six-year civil war has seen it divided into a patchwork of fiefdoms but in IS the disparate members of the SDF found a common enemy.
The alliance is estimated to command about 30,000 fighters -- some 25,000 Kurds and 5,000 Arabs.
The Syrian Kurdish People's Protection Units (YPG), which had already dealt IS several defeats including seizing the key border town of Tal Abyad, is the alliance's backbone.
Along with the Kurdish female Women's Protection Units, the SDF includes Arab factions, Syriac Christian fighters and Turkmen units.
The alliance has often emphasised its Arab component, despite allegations that they are effectively a junior partner in the force.
After launching the coalition air war against IS in Syria and Iraq, Washington struggled to find a reliable partner on the ground.
A much-touted $500-million programme to build a rebel army to fight IS collapsed after many candidates failed the screening process and one group surrendered equipment to an Al-Qaeda affiliate.
But since its creation the SDF has benefited from strong US backing, including weapons drops and air strikes in support of its operations.
Shortly after the SDF was formed, the White House announced the first sustained deployment of US special forces to Syria, reversing a longstanding refusal to put boots on the ground.
Some 50 special operations personnel were deployed in northern Syria, and the number has now grown to around 500 US troops, though not all are from special operations.
US officials including Washington's envoy to the coalition Brett McGurk and the head of Central Command have met senior SDF commanders on visits to northern Syria.
And in June, Washington announced it would supply weapons directly to the YPG, over objections from ally Turkey.
In November 2016, the SDF announced its "Operation Wrath of the Euphrates" aimed at eventually ousting IS from Raqa province, including its de facto Syrian capital Raqa city.
Over the following months, the alliance has gradually closed in on the city, first sweeping down and capturing territory to the north before closing in from the east and west.
A key battle was for the town of Tabqa and the neighbouring dam, around 40 kilometres (25 miles) west of Raqa.
That operation began in late March, with US forces airlifting SDF troops behind IS lines.
After heavy fighting, and fears for the integrity of Tabqa dam, the SDF seized the town and the dam on May 10, severing a key route out of Raqa.
The rise of the SDF has raised deep concerns in Turkey, which considers the YPG to be a "terrorist" offshoot of the Kurdistan Workers' Party (PKK) that has waged an insurgency inside Turkey since 1984.
Turkey launched an operation inside Syria on August 2016 alongside allied opposition forces and managed to retake the IS stronghold of Jarabulus and the symbolically important town of Dabiq.
The operation also sought to check the advance of Syria's Kurds, and Turkish forces have carried out air strikes against YPG positions.
But Ankara failed to convince Washington to hold off on the Raqa operation and find an alternative partner to the SDF.
It reacted with fury to Washington's decision to arm the YPG directly, despite US assurances that the weapons supplies would be limited and monitored.