How does the U.S. decide which Russians to throw out of the country?

Robert Windrem, NBC

Image from article, with caption: Army officers remove the bench on March 23 where Sergi Skripal and his daughter were found in Salisbury, England, on March 4.

Russia violated an unwritten rule of espionage, officials say: No assassinations on foreign soil.

Excerpt:

Richard Haas, the president of the Council on Foreign Relations [tweeted that] expulsions are "less than optimal" and "will likely lead to Moscow responding in kind," meaning expulsions of U.S. diplomats — an accurate prediction of what occurred Thursday.

Haas suggested instead an "asymmetrical response, i.e., targeted economic and travel sanctions, increased public diplomacy [JB emphasis] vs Putin, etc." Otherwise, he suggested the costs will fall mainly on the U.S. in the form of reduced capabilities in Moscow and elsewhere inside Russia. ...