Iran: Russia blocked Iranian air-to-ground weapons on MiG-29s through the Natasha system, required Russian advisers to confirm the serial number of every spare part, and even reverse-engineered parts could not be installed. After the UN arms embargo, to which Russia also joined, Natasha became a serious obstacle for Iran. Russia has been tougher with Iran than with any other country, because Iran retrenched weapons after American advisers left and Russia did not want its arms to remain operational if relations soured.