A common expression we used during the negotiations over the Iran nuclear deal of 2015 was cautious optimism. It aptly sums up what will be felt in Brussels, Paris, London and Berlin on reports that President-elect Biden plans to re-join the Joint Comprehensive Plan of Action (JCPOA), as it is formally called. It won’t be easy for a Biden administration to get back to the deal that President Donald Trump withdrew the U.S.