take up arms

Also, take up the cudgels. Become involved in a conflict, either physical or verbal, as in The Kurds took up arms against the Iranians at least two centuries ago, or Some believe it's the vice-president's job to take up the cudgels for the president. The first term originated in the 1400s in the sense of going to war. The variant, alluding to cudgels as weapons, has been used figuratively since the mid-1600s and is probably obsolescent.

The verb has one meaning:

Meaning #1: commence hostilities
  Synonyms: go to war, take arms