The USA government does not have to do anything in secret.

They just take over the mine and let the miners stay where they are.

Where did you get the idea from that they have to "move" them and it has to be "secret"?

If everybody panic sell the news about it, the USA government would be happy...not sad.

The USA government could attack any PoW blockchain 365 days a year with the "nationalized" miners.  

 Correct. That is why no jurisdiction should host more than some % of total hashrate. The faster governments understand this, the better.