President-elect Joe Biden has said he plans for the U.S. to re-enter the Iran nuclear deal he helped establish under the Obama administration in 2015. WSJ’s Gerald F. Seib explains why doing so won’t be as simple as it sounds. Photo: Abedin Taherkenareh/EPA/Shutterstock Friday’s killing of one of Iran’s most important nuclear scientists has dealt a setback to the country’s nuclear program, which had staged a partial rebound after the 2015 nuclear deal brokered under President Obama unraveled. Yet, the U.S. and its allies still face major hurdles in trying to constrain Tehran’s nuclear ambitions, current and former officials said. The…