By Patricia ZengerleWASHINGTON, April 1 (Reuters) - President Donald Trump threatened on Wednesday to pull the United States out of NATO due to its European members' refusal to send ships to unblock the Strait of Hormuz near Iran, as he intensified his denunciations of the military alliance.Experts say it is not clear whether Trump could act unilaterally to leave the 77-year-old trans-Atlantic coalition, even though he frequently makes major decisions without congressional approval, some of whi...