join forces - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Act together, combine efforts. For example, The public relations people joined forces to get better coverage for their candidates. This expression originally referred to combining military forces. [Mid-1500s]