Recalled to develop technology and tactics to deal with the alien threat.

And let's not forget the various alien threats out there.

After all, no Earth explorer had ever encountered any other alien threat.

Do we have some knowledge of this alien threat after all?

It's very important now that this new alien threat has arrived.

"It will be difficult enough to prosecute a war against an alien threat we don't understand."

They died facing an alien threat that put machines above living beings.

"What has this to do with the so-called alien threat?"

The first 200 pages involves just the build up to the idea that there could be an alien threat.

Fear of an alien threat meant jobs in a new weapons industry.