US President Donald Trump has demanded help from allies to reopen the Strait of Hormuz, which usually carries about a fifth of global energy supplies, but even if he can assemble a major coalition, it could prove very hard to end Iran’s blockade.Iran lies along one side of the narrow strait and has responded to the US-Israeli attack from February 28 by using drones, missiles and mines to make the vital waterway unsafe for the colossal oil and gas tankers that slowly traverse it each day.For all the latest headlines follow our Google News channel online or via the app.Why has Iran cut off the strait now? When a commander in Iran’s Islamic Revolutionary Guard Corps warned in 2011 that cutting off the strait would be “easier than drinking a glass of water,” the threat to the strait had already been made many times before. In the years since, the Guards have continued to warn they could close it, including during tensions over sanctions and Iran’s nuclear program