in het geweer komen

Figurative sense for an earlier, now obsolete literal sense “to get under arms”, in which the sense of the word geweer as a verbal noun of the verb (zich) weren (“to defend oneself”) has also become obsolete.

• IPA^(key): /ɪn ɦət ɣəˈʋeːr ˌkoːmə(n)/, /ɪn ət xəˈʋeːr ˌkoːmə(n)/

in het geweer komen

1. (intransitive) to come into action; to oppose; to protest; to resist [with tegen ‘against’]

   De actievoerders kwamen in het geweer tegen het kappen van de bomen.

   The activists came into action against the cutting of the trees.

The verb komen imparts an inchoative sense, signaling the transition from inaction to action.