The Robotech Defense Force (RDF) was a military force that was part of the United Earth Forces. It was created specifically to defend Earth against the alien presence discovered with the crash of the giant alien spaceship in 1999.
The force made heavy use of Robotechnology and was equipped with advanced weapons systems such as Veritech fighters, VF-1 Valkyrie and VF-4 Lightning and Destroid combat mecha, MBR-04-VII Tomahawk, MBR-07-II Spartan, ADR-04-X Defender, HWR-00-II Monster and SDR-04-XII Phalanx.
Although unclear after the retcon brought about by Robotech: The Shadow Chronicles, it seems that the RDF was gutted to provide personnel and resources for the Robotech Expeditionary Force (REF). The Robotech: Roleplaying Game (book number four), published by Palldium Books, makes reference that only two RDF bases did in fact remain on Earth after the departure of the REF. This would be in keeping with the fact that the United Earth Forces would need the Army of the Southern Cross to take over the defense of Earth after the Pioneer Mission left.