The aliens invade earth so that they can take it back and start again
They use a giant ancient demon-summoning altar in Australia to create large alienesque giants
The 'Leviathan' in Australia is beaten, but with large casualties
Eartheans figure out that the altars are actually really old alien artifacts that were used in ancient times after the aliens first left to 'summon demons' which was actually when a sacrifice was used to create a large 'demonic' unit. The ritual was pushed down by the church and forgotten, until the aliens returned to reclaim their old planet.
The humans found more altars all over Europe, and attempted to take them off the aliens, so that they could create their own 'demons'
The game begins. You are a general in charge of a European army, attempting to take over an altar, so that you can create a 'Titan'... and claim an area for humanity.