Word for enemy becoming an ally?

Does a word (or short phrase) exist to represent an enemy becoming an ally?

Solution 1:

rapprochement:

noun

(Especially in international affairs) an establishment or resumption of harmonious relations:

The entry from etymonline.com:

"establishment of cordial relations," 1809, from French rapprochement "reunion, reconciliation," literally "a bringing near," from rapprocher "bring near," from re- "back, again" (see re-) + aprochier (see approach (v.)).

The specific purpose and extent of the approach to harmony is not specified in the word. At the end of every primary election campaign in the USA, the opponents develop some level of rapprochement to work together against the larger political opponent. After every successive revolution for independence, Great Britain has revealed its international savvy in rapprochement with its former colonies. During WW II, Germany, Italy and Japan overlooked their ancient hostilities and current ideological differences to establish an axis of world domination, while the United States, the Soviet Union, and Great Britain formed The Grand Alliance in similar rapprochements.

Solution 2:

Reconciliation:

an act of reconciling, as when former enemies agree to an amicable truce.

dictionary.reference.com

Example:

Adoption results in a child becoming a son and heir, redemption results in a slave becoming a servant and reconciliation results in an enemy becoming a friend.

_{^{The English Connection: The Puritan Roots of Seventh-Day Adventist Belief by Bryan W. Ball}}

Solution 3:

Shakespeare's Strange bedfellows comes to mind. I'll admit that it's maybe not ideal here, though.

Unlikely companions or allies; often used in the phrase “politics makes strange bedfellows.”

[Dictionary.com]

You can also use

make common cause (with)

To cooperate, to enter into an alliance for a shared goal.

[Wiktionary]