By Jonathan Landay, Phil Stewart and Erin BancoWASHINGTON, March 16 (Reuters) - President Donald Trump was warned that attacking Iran could trigger retaliation against U.S. Gulf allies despite his claims on Monday that Tehran's reaction came as a surprise, said a U.S. official and two sources familiar with U.S. intelligence reports. Pre-war intelligence assessments did not say that Iran’s response was "a guarantee, but it certainly was on the list of potential outcomes," said one source, who lik...