DOHA, Qatar — U.S. Secretary of State Rex Tillerson took the Trump administration’s case for isolating and containing Iran in the Middle East and beyond to two Gulf Arab nations on Sunday, pushing for Saudi Arabia and Iraq to unite to counter growing Iranian assertiveness. He also called for a quick resolution to the ongoing crisis between Qatar and its Arab neighbors, which he said was unintentionally bolstering Iran. In Saudi Arabia and later Qatar, Tillerson denounced Iran’s “malign behavior” and urged nations of the region and elsewhere, notably Europe, to join the administration to halt any business they do with Iran’s powerful Revolutionary Guard Corps.