Does a word (or short phrase) exist to represent an enemy becoming an ally?
(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.
an act of reconciling, as when former enemies agree to an amicable truce.
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
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.”
You can also use
make common cause (with)
To cooperate, to enter into an alliance for a shared goal.
You could also use detente, though the meaning is somewhat similar to rapprochement:
noun the easing of hostility or strained relations, especially between countries
Foe turned friend would perhaps answer.
This is actually something Benjamin Franklin did, by asking an opponent for the loan of a rare book, in the nicest possible way, while using his reputation of having discerning literary tastes.
Furthermore a slightly related quote is:
“An injured friend is the bitterest of foes.” – Thomas Jefferson.Backwards, I know
Turncoat is one word for it. It has a negative connotation - it implies the person cannot be trusted, as they have betrayed their original cause once already.
A person who deserts one party or cause in order to join an opposing one.
"they denounced him as a turncoat"
Defector is also a possibility, doesn't have the negative connotation, at least as strongly.
A person who has abandoned their country or cause in favour of an opposing one:
"staff interviewed escapees and defectors to the West"
Here is a Google Ngram comparing the usage of defector from the perspective of the injured party
and the perspective of the advantaged party
enemy of my enemy, as in “the enemy of my enemy, is my friend,” is a workable idiomatic phrase.
Common interests are usually what motivate such dramatic changes in status i.e., former adversaries conclude that their interests are better served by mutual cooperation than by competition, due to emerging threats or opportunities.
The enemy of my enemy is an ancient proverb which suggests that two opposing parties can or should work together against a common enemy. The earliest known expression of this concept is found in a Sanskrit treatise on statecraft dating to around the 4th century BC, while the first recorded use of the current English version came in 1884. Some suggest that the proverb is of Arabic origin. see, Wikipedia
You could say the person in question had a "change of heart".
if someone has a change of heart, they change their opinion or the way they feel about something
The wrestling phrase 'Heel-Face turn' comes to mind.