When I was a kid, I saw a black-and-white movie on TV. An alien spaceship arrives at Earth, and flies around, generating "a million degrees of heat." This is mentioned very specifically. One million degrees.

It then flies around, melting cities. Paris, Rome, London, Washington, DC maybe. In my memory, this was done with special effects that rivalled INDEPENDENCE DAY, though I suspect a repeat viewing would be sobering.

Cities riot, sort of like the end of (the original) WAR OF THE WORLDS.

In the end, a brave scientist sacrifices himself to load a radioactive warhead onto a rocket, killing himself in the process, leaving his Best Girl to grieve for him.

Shortly before the end, I think we see the interior of the spaceship, maybe a bit of a control panel, and a bit of the alien itself, which is an eyeball on a tentacled stalk, or something. The film never gets into the motivation of the alien, except that, if your spaceship can generate a million degrees of heat, of course you're going to go lay waste to Earth.

The rocket intersects the approaching spaceship, and the Atomic Bomb is once again the salvation of Mankind. Even a spaceship radiating a million degrees of heat can't withstand an atomic blast, apparently.

I've been trying for years to find out what movie this was (Admittedly, I've not tried all THAT hard, but every couple of years I'd do 5 minutes of Google searching, or post a message on a movie newsgroup, or whatever).

I knew it wasn't THE SPACESHIP THAT GENERATED A MILLION DEGREES OF HEAT or THE REALLY VERY HOT SPACESHIP.

But this time, success!

It's THE LOST MISSILE (1958) starring Robert Loggia, and cowritten by Jereme Bixby, who did some TWILIGHT ZONE and STAR TREK scripts.

Odd that I referenced INDEPENDENCE DAY, since Robert Loggia is also in that. But I think I'd have to rate THE LOST MISSILE as my second-favorite Robert Loggia movie (The first being that Mafia-vampire comedy where La Femme Nikita is naked as a jaybird---INNOCENT BLOOD).