WASHINGTON—The United States will do what it sees as necessary to defend its interests after a rocket attack last week against Iraq’s Ain al-Sada airbase, which hosts American, coalition, and Iraqi forces, U.S. Defense Secretary Lloyd Austin said on Sunday. Speaking on ABC’s “This Week” program, Austin said the United States is urging Iraq to quickly investigate the incident at the base located in western Anbar province and determine who was responsible. U.S. officials have said the incident fit the profile of a strike by Iran-backed militia. “We’ll strike, if that’s what we think we need to do, at a time and place of our own choosing. We demand the right to protect our troops,” Austin said. Asked if Iran had been given a message that U.S. retaliation would not constitute an escalation, Austin said that Iran is fully capable of assessing the strike and U.S. activities. “What they should …