It's pretty much standalone. The background can be summed up as:
WW2 ends in 1944 when Japan surrenders conditionally and the Americans nuke Berlin. The Cold War proceeds as normal.
Humans explore space much more quickly than us and build a moonbase in the 50s, but their first Mars probe is abducted by alien monsters, the humans fight the aliens on the moon for a while before they arrive on Earth in Western China in 1973. The Chinese and the Russians have the advantage for a time until the aliens suddenly roll out a perfect laser-based air defense system that totally denies air superiorit…