"For years, the combatants crossing Ukraine’s borders to pick up arms were joining the pro-Russian forces that had taken control of a territory in eastern Ukraine. Since 2014, more than 17,000 fighters from more than 50 countries have joined the Russian-backed forces there, according to the Counter Extremism Project, a monitoring group. Last week, Ukraine’s military said another detachment of foreign mercenaries had arrived in the east, part of what Kyiv says is a plan to stage provocations to justify Russia’s invasion."