The United Defense Force is a joint operating organization that was created by the United States, United Kingdom, Russia, Canada and China; the organization was created to stop the mimics and defeat them once and for all. The closest base is in London to strike on Normandy to start operation downfall.
After the invasion of Europe, countries around the world, feeling that after Mimics are done with Europe they will continues to expand into other mainlands, came together to find a way to stop them. Consisting of United States, United Kingdom, Russia, Canada and China, UDF boast the largest army in the world. With their combined efforts, UDF was able to develop new warfare technology, like Combat Jackets or Quad Tiltrotors.