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separate peace

n. An agreement to end hostilities with an opponent, while remaining hostile with that opponent's ally.

Wikipedia
Separate peace

The phrase "separate peace" refers to a nation's agreement to cease military hostilities with another, even though the former country had previously entered into a military alliance with other states that remain at war with the latter country. For example, at the start of World War I (1914–1918), Russia was a member, with the United Kingdom and France, of the Triple Entente, which went to war with the Central Powers formed by Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria. After the fall of Russian monarch Nicholas II and the rise to power of the Bolsheviks, Russia defaulted on its commitments to the Triple Entente by signing a separate peace with Germany and its allies in 1917. This armistice was followed on 3 March 1918 by the formal signing of the Treaty of Brest-Litovsk.