The Obama administration carried out the exile of Russian diplomats as a revenge of Russia's hacking against the previous presidential election.
Sydney Morning Herald: US expels 35 Russian diplomats, closes two compounds over election hacking
A total of 35 Russian diplomats were forced to return home from the Russian embassy in Washington. Also, two Russian compounds were shut down.
It was apparent that Russian government repeatedly tried to control the result of the US Presidential vote. Donald Trump, the Presidential-elect, is friendly to Russia for unknown reasons, while Hilary Clinton has a strong policy against Russia. Russia itself admitted it conducted some hacking activities on the US government. It is still unclear, however, Russia's attempt was successful.
Responding the US declaration, Russia requested the canceling of the decision, as claiming the sanction is illegal. But, Putin commented that Russia would not take revenge to the US regarding this issue.
The New York Times: Vladimir Putin Won’t Expel U.S. Diplomats as Russian Foreign Minister Urged
In my opinion, Obama administration took a wrong way to address this problem. First, there is no evidence suggesting Russia's interference has influenced the outcome of the Presidential vote. Otherwise, the US would resume the vote. Thus, this sanction is kicking-the-cat for the defeated Democratic Party. Second, Russia seems indifferent to the movement of the US. Obama administration is a Lame duck. Putin only has to wait for the inauguration of Trump. And finally, it is highly possible every nation frequently take cyber attacking. The CIA in the US is not an exception. If the US government was compromised by Russian cyber team, simple the US is too weak to survive the modern age.