We've known about this battle for years, but have had very little information on what exactly happened. Finally, here's an article that has a lot of detail of the battle. Details emerge of Army Special Forces’ battle with Russian mercenaries Very good read!
In 2015, American forces were in Syria. Their precise purpose was unclear. Supposedly they were trying to fight ISIL terrorists, but America was also probably trying to overthrow the Syrian government at the time. Russia viewed this as a possible American attack on Syria, and sent in mercenary forces to protect the Syrian government, as well as also to fight ISIL. It was a three-way war, with America backing the revolutionaries in Syria, Russia backing the Syrian government, and then ISIL (terrorist militants) trying to carve out an Islamic state in Eastern Syria and Northern Iraq. Some believe the U.S. (under President Obama and Secretary of State Hillary Clinton) supported ISIL to be able to support the policy of trying to topple the Syrian government, although it's hard to find incontrovertible proof that was the case. Towards the later stages, at times the U.S. did fight ISIL forces in Syria. The U.S. and Russia mostly tried to avoid a direct conflict with each other, despite their agendas and alliances in the region conflicting. Both the U.S. and Russia claimed they were mostly there to fight the terrorists, but this was probably more convenient political propaganda. Some might view the presence of both American and Russian forces in Syria as a little bit like a war-by-proxy, reminiscent of many Cold War era conflicts.
