Four years of economic warfare against Iran and its allies have barely changed their behaviour.

“Maximum pressure”, as Mr Trump calls it, has been a tactical success. In April Iran’s oil exports dipped as low as 70,000 barrels per day, compared with 2.5m two years before. (Exact numbers are elusive because much of Iran’s oil trade is now done in secret.) The rial, Iran’s currency, has lost 85% of its value. Yet economic pain has not brought political change. Sanctions have not compelled Iran to halt its support for militias nor convinced Bashar al-Assad, the Syrian dictator, to stop bombing his people. Sanctions may be an alluring tool for presidents. They are inexpensive, bloodless and largely up to executive discretion. But they often do not work.

Sanctions can be effective when they have broad international support, achievable demands and are targeted at firms and people that need to trade and travel. A multilateral embargo on Iran led to the deal in 2015 that restricted its nuclear programme. Sanctions on Rusal, a Russian aluminium giant, forced a Kremlin-backed oligarch to surrender control of the firm.

Mr Trump’s maximum-pressure campaign, however, fulfils none of these criteria. For a start, many of his sanctions are unilateral, and some have begun to fray. Iran’s oil exports have climbed from their nadir in April, perhaps to as high as 1m barrels a day this autumn, as some countries (particularly China) have defied American threats and snapped up discounted crude.

https://www.economist.com/middle-east-and-africa/2020/11/26/donald-trumps-sanctions-in-the-middle-east-have-had-little-effect