The European Union’s announcement that it would establish a special payments channel to maintain economic ties with Iran sent a clear message to Tehran and Washington: Europe is intent on trying to save the 2015 nuclear deal.

The special-payment channel is intended to bolster those in Iran arguing Tehran should keep honoring the deal’s terms. Iranian officials have been warning for months they would follow the Trump administration in quitting the agreement if the economic benefits of the 2015 deal stop flowing.

Under...