Invasion on the Homeland

It is the early 1970s. The Vietnam War is drawing to a close as the United States tries to recover an embarrassing defeat from the Tet offensive few years prior, been putting men on the moon, and the near height of the Cold War against Russia. As the higher-up of the Pentagon and President Nixon prepare for one massive counter-attack against the North-Vietnamese, the Gate appears in the middle of Washington D.C. and a force from the other side comes out and attack the District. As the outworlders failed, POTUS prepares for another all out assault by recalling all troops from Vietnam to immediate deployment on the other side of the Gate, unknowingly waiting what's ahead of them.