Need help to explain "set a-rattling the sabers of"

It looks like a combination of two standard phrase forms:

1. set a-verb, which means simply to start the operation of the verb.

   For example, to set a-flutter is to start {something} fluttering. This form often seems to diminish or trivialise the verb.

2. saber rattling (or sabre rattling), which means to deliberately telegraph a threat, or otherwise communicate a threatening posture

So, while you can find plenty of examples of both in isolation, this particular combination may be original.

Sabre-rattling (or saber in American English) is an idiom with the sense of 'rattling one's sword in its scabbard as though threatening to draw it' https://dictionary.cambridge.org/dictionary/english/sabre-rattling

Here, the writer has turned the phrase round. The meaning seems to be 'made them behave in a threatening manner' rather than a reference to actual military action.