Space Invader

Identity and Technology
The space invaders are a war-like race who are in constant battle with humanity. They travel in swarms which slowly descend towards their ground based enemies, all the while dropping projectiles, it is likely that this projectiles are missiles since the slowly fall and explode on impact. They are an even match for humanity, for while they far out number them, humanity are capable of constructing shields which act as temporary defences and are also able to evade the incoming missiles much more effectively than the invaders and they appear to have simalair weaponry.

The origin of the invaders or thereason they are at war with the Human race is unknown. So is the appearance of the invaders since it is not known whether they are inside the descending objects, which are therefore spacecraft, or if they are the objects! They have huge assault fleets which continually fire at the ground based human vehicles. They also have larger craft which travel over every so often but without attacking. Possibly, these are scouts which may be giving up to the minute knowledge of events on the battlefield back to the invaders centre of command. An altenative possibility is that it is the centre of command and it is moving over to co-ordinate the craft, however, if the object is destroyed, the assault crafts continue to function which seems to dicredit that theory.

Update: As it turns out the Invaders are an imperial race, as they have invaded many other planets in the past.

Strategy
The invaders have utilised their ability to must wave after wave of expendable soldier, they are ultimately impossible to defeat! If a human ground defence destroys ship after ship, sooner or later, they will make a mistake. it doesn't matter how long it takes. Eventually, one of the ships won't yet have been destroyed as this mistake is made and the invaders win. The only way that humanity have a chance of survival therefore, is to keep there guard up!