The U.S. leveled its most serious allegation of a treaty violation against Russia on Monday, accusing it of breaking a 1987 arms treaty by testing a ground-launched cruise missile. President Obama apparently wrote Putin a letter conveying the charge, which will be made public in an upcoming State Department report. Meanwhile—although sanctions haven’t deterred Russia from supporting separatists in Ukraine so far—the United States and European Union hope a fresh batch will do the trick. The U.S. is joining the EU in hitting Russia with new economic sanctions this week. According to Tony Blinken, a White House national security adviser, Putin may be preparing to send rocket launchers to the east to help rebels.