US President Donald Trump briefly laid out his case for a possible attack on Iran in his State of the Union speech to Congress on Tuesday, saying he would not allow the world's biggest sponsor of terrorism to have a nuclear weapon.

Even while assembling a massive military force in the Middle East, Trump has done little to explain to the American public why he might be leading the US into its most aggressive action against the Islamic Republic since its 1979 revolution.

In his speech, Trump pointed to Tehran's support for militant groups, its killing of protesters and the country's missile and nuclear programs as threats to the region and the United States.