Wiktionary
vb. To cooperate, to enter into an alliance for a shared goal.
Usage examples of "make common cause".
And if you make common cause with these, ride swift, Green Lady, for they need all the aid possible.
Wendell Phillips saw Garrison in the hands of a Boston mob, and that experience determined him to make common cause with the martyr.
It would not be the first time that Ned had been forced to make common cause with a man he despised.
At this doleful news Pentaquod asked what his little tribe could do, and Henry suggested that they move south and make common cause with the Nanticokes.
Finally, in 1869, the Minnesota Sioux, goaded by many wrongs, arose and murdered many of the settlers, afterward fleeing into the country of the Unkpapas and appealing to them for help, urging that all Indians should make common cause against the invader.