The tit for tat with Iran continues.

After rioting outside Britain’s embassy in Tehran, Prime Minister David Cameron expelled Iranian diplomats in London and shuttered their embassy. In the United States, the senate unanimously approved legislation that would penalize any foreign bank that does business with Iran’s central bank. The EU, for its part, increased sanctions.

Over the weekend, the Iranians claimed to have shot down a U.S. drone that was on an intelligence gathering mission.

Ahmad Sadri, a professor of Islamic world studies at Lake Forest College, parses the latest developments.