More often than not, when an alien species invades they do so alone. Or they do so and bring their pets/creations in.
I remember back when I played Xcom: Enemy unknown, you face an alliance of aliens all intent on invading and taking over earth.
Is this the first time an alien alliance or a group of separate alien species are seen invading earth?
For definition purposes:
Invasion (sometimes interchangeable with Conquest) is a military offensive consisting of all, or large parts of the armed forces of one geopolitical entity aggressively entering territory controlled by another such entity, generally with the objective of either conquering, liberating or re-establishing control or authority over a territory, forcing the partition of a country, altering the established government or gaining concessions from said government, or a combination thereof.
My emphasis on the important part.