The cryptic job listings began appearing online early this year. The tasks were menial — posting fliers or hanging signs in public spaces — and the pay meager. But for a handful of refugees from eastern Ukraine, the promise of quick cash was too good to pass up. Respondents soon realized there was a catch: The jobs involved distributing pro-Russian propaganda on behalf of an anonymous employer. For those willing to complete the assignments anyway, the work then took a more ominous turn.  Within weeks, recruits were tasked with scouting Polish seaports, placing cameras along railways and hiding tracking devices in military cargo, according to Polish investigators. Then, in March, came startling new orders to derail trains carrying weapons to Ukraine. Polish authorities now believe that the mysterious employer was Russia’s military intelligence agency, the GRU, and that the foiled operation posed the most serious Russian threat on NATO soil since Moscow launched its invasion of Ukraine last year. Russia’s objective was to disrupt a weapons pipeline through Poland that accounts for more than 80 percent of the military hardware delivered to Ukraine, a massive flow that has altered the course of the war and that Russia has seemed helpless to interdict, according to Polish and Western security officials.