Separate peace
A separate peace is an agreement by one nation to cease military hostilities with another, even though allies of the country remain at war with the latter country. For example, at the start of the First World War, Russia was a member of the Triple Entente. After the February Revolution and the October Revolution, 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 March 3, 1918, by the formal signing of the Treaty of Brest-Litovsk.
During the Second World War, after 1941, when the Soviets were allied with the British and the Americans, to the end of the war in 1945, both sides suspected the other of seeking separate peace with Germany, though this did not occur.
An earlier important example is the Franco-Dutch War of 1672, which France and England entered together, but from which the English withdrew unilaterally by a separate peace with the Dutch, the Treaty of Westminster (1674).