The War in Space (1977) on Netflix

Plot summary

In 1988, UFO sightings are reported across America. A space station crew reports a large galleon-like vessel before communications go silent. The United Nations Space Federation orders Professor Takigawa to finish Gohten, a defense ship he abandoned three years earlier. Miyoshi, a UN team member, brings the order, but Takigawa refuses. After an incident at his home, Commander Oshi takes command of Earth's defense forces, and Takigawa is ordered to complete the vessel. Alien forces based on Venus then strike major cities. The Gohten team reaches the island where the ship is housed, and with the vessel finished, they prepare to launch against the invaders.