The basic premise was space Marines defending Earth against hostile aliens.

He certainly was one of the most important members of the fighting troop which at the present time had to defend Earth.

The player characters must defend Earth from this alien threat and restore human civilization.

It felt strange to be defending Earth.

There's no point in trying to defend Earth at all.

A 16 year old wizard who has to defend Earth from the Nekross.

He was not involved in defending Earth from the Borg.

We must save those planes and equipment if we have to defend Earth.

Its prime directive is to defend Earth against extraterrestrial threats.

The Federation will be lucky if they can defend Earth.