In this pool photograph distributed by the Russian state agency Sputnik, Russian President Vladimir Putin shakes hands with U.S. President Donald Trump's envoy Steve Witkoff during a meeting in Saint Petersburg on April 11. GAVRIIL GRIGOROV/Pool/AFP via Getty Images

WASHINGTON — While Donald Trump brags about his “maximum pressure” campaign to impoverish Iran and force its leaders to give up its nuclear program, there remains one source of revenue that the president appears to be OK with: Iran’s sale to Russia of deadly drones, missiles and technical expertise to help slaughter Ukrainian civilians.