What happened near Khasham before 12 February 2018?

What had happened in eastern Syria in the days before 12 February 2018, and why did it matter to Russia and the United States? The reports available by that date described a deadly clash near the town of Khasham, in the Deir ez-Zor Governorate.

According to the account of the Battle of Khasham, a force of 500 Syrian pro-government troops, backed by tanks and artillery, launched a ground assault on a local headquarters of the Syrian Democratic Forces. The attack took place on 8 February 2018. More than 100 government fighters were killed, among them Russian private military contractors of the Wagner Group.

The United States Central Command had described the episode in a statement dated 7 February 2018. It said it had killed more than 100 pro-government troops in what it called self-defence strikes. These were carried out 8 kilometres (5 miles) east of the Euphrates de-confliction line, which had reportedly been agreed with Russia.

By 12 February 2018, then, the reports tied together a ground assault on a Syrian Democratic Forces position, US strikes in response, and the deaths of Russian contractors among the Syrian government's fighters.