When the Romans returned to defend their own land, the Britons called upon from modern day to help them defend themselves from the invading Celts.

When the Romans returned to defend their own land, the Britons called upon mercenaries from modern-day Germany to help them defend themselves from the invading Celts.

Explanation:

The Celts had migrated to the British Islands around 800-600 BCE

However, in 55 BCE Britain was invaded by Julius Caesar and claimed as a Roman province.

In 407 AD Roman troops leave Britain to fight the barbarian invasions in Rome, which left no military structure behind.  

So, when the northern Celts invaded them to recover the land, the Britons summoned foreign mercenaries.

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