"The Trump administration said the order issued Thursday was in retaliation for the Kremlin's "unwarranted and detrimental" demand last month that the U.S. substantially reduce the size of its diplomatic staff in Russia."
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Except that Russia's reduction of USA diplomatic staff in Russia was a response to Obama illegally barring Russia from access to two of its diplomatic compounds in the USA, while also expelling 35 Russian diplomats.
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The USA acted first, without provocation. And I expect that Russia will respond to each further non-provoked and hostile action that the USA might make towards Russia.