"Ghidorah, the Three-Headed Monster" released in U.S in 1965

For an early movie, this one had a surprisingly good (i.e. not nearly as super-campy) plot.  A princess from a small country leaps from a plane just before it is blown up by assassins.  She re-appears in Japan possessed by a Martian, showing up at sites where Godzilla and Rodan are hibernating. A strange meteorite falls to Earth.  As it happens, the meteorite contains King Ghidorah, the monster that destroyed the Martian's planet. 

To save the Earth, the Cosmos and Mothra have to convince Godzilla and Rodan to stop attacking each other and join forces to defeat King Ghidorah.