Make common cause with somebody
Definition 1 of 1
Pro Tip 1/3Shared Goals
Use when individuals or groups work together due to shared objectives, not necessarily lasting friendship. 
Environmentalists and fishermen made common cause to protect the bay.
Pro Tip 2/3Temporary Alliance
The phrase often implies a temporary alliance, not a permanent partnership. 
The opposing political parties made common cause to pass the emergency bill.
Pro Tip 3/3Mutual Opposition
It's commonly used when groups unite to oppose a mutual threat or enemy. 
Several nations made common cause to combat climate change.