Re: "The United States will do what we must to prevent…"
He has used "will do what we must" to make it clear that he, as the leader of the US, along with presumably the government and the people will do what needs to be done by them. He could have replaced "The US" with "We":
We will do what we must to prevent Iran from obtaining ...
It would be clear to a listener that he is talking about himself, his government, and the country he leads.
"The US will do what must be done" is OK but doesn't give the same emphasis, but your "what it is a must" is incorrect English.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.