In the book alien drone like ships come to Earth and invade Earth for its metal.
The ships use some tractor beam magnet type to gather the metals. They also remove metal from people, like braces, hip replacements and metal plates in a person's skull.
They are able to capture one of the drones and "hack" into and and gain control. They somehow gain a small fleet of alien ships and paint it like the American flag.
This is Von Neumann's War by John Ringo and Travis S. Taylor which features small alien probes/devices reaching Earth via Mars and devouring metal to create more of themselves.
The strip all metal in their path, including metal fillings etc. There is a scene where the government realise that danger and send advisories about dental work.
Cady realized her concern. Some of the horrible images from his and the major’s trip to Paris of soldiers being decapitated flashed in his mind. What if a bot got close enough to pull the metal out of this poor kid’s mouth? If the damn thing pulled the metal straight out of her mouth she would likely lose some teeth and have her lips, and tongue ripped to shreds.
As for the American flag...
Two probes, their surface now shifted from glittering steel to bands of red, white and blue, held the coffin a meter off the ground as honorary pallbearers walked on either side.
