Its cities lay in ruins, foreign armies divided and occupied its territories, millions of people were in flight and without shelter; the entire population faced starvation. (firstthings.com)
From an anthropologist point of view, this behaviour can trace all the way back to ancient Greece, where discredited generals borrowed foreign armies to attack his own city. (politics.stackexchange.com)
If you successfully achieve a Fame, Conquest, or Kingdom Victory while playing a long campaign, one of three powerful foreign armies (the Normans, Danes, or Norse) will appear and try to take your lands. (wccftech.com)