The cracks in the relationship between President Trump and Defense Secretary Jim Mattis that led to the Pentagon chief’s resignation were apparent from the start.

During his 2017 Senate confirmation hearing, the retired Marine general — who said Thursday he would leave the Pentagon on Feb. 29, 2019 — backed the U.S. staying in the Iran nuclear deal. That came even as President-elect Trump said he considered the pact disastrous and angled to fulfill a core campaign promise to dismantle it.