Student or Learner
On Tuesday, US President Barack Obama told the UN Assembly that time was running out to find a diplomatic solution to the Iranian nuclear issue. "The United States will do what we must to prevent Iran from obtaining a nuclear weapon," he said.
Source:PM: On Yom Kippur, Iran tyrant called for our demise - Israel News, Ynetnews
I am not familiar with this kind of Grammar for which I think the right structure should go this way, The United States will do what must be doneů. or what it is a mustů..
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 - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.