(CNN) President Trump says he's sending the military to help guard the border until his long-promised wall is built.

Key details about the President's plans to deploy National Guard troops have yet to be announced -- including how many troops he's requesting, where they'll go, how long they'll be there, exactly what they'll do and how much it will cost.

But the idea of deploying the National Guard to help enforcement along the border is nothing new.

President George W. Bush and President Barack Obama both did it, in two operations that cost a total of more than $1.3 billion.

Critics called those deployments costly and inefficient . Supporters said they helped US Customs and Border Protection fill in gaps and step up enforcement.

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