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

War

War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

 with the latter country. For example, at the start of World War I

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

 (1914–1918), Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 was a member, with the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 and France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, of the Triple Entente

Triple Entente

The Triple Entente was the name given to the alliance among Britain, France and Russia after the signing of the Anglo-Russian Entente in 1907....

, which went to war with the Central Powers

Central Powers

The Central Powers were one of the two warring factions in World War I , composed of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire, and the Kingdom of Bulgaria...

 formed by Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, Austria-Hungary

Austria-Hungary

Austria-Hungary , more formally known as the Kingdoms and Lands Represented in the Imperial Council and the Lands of the Holy Hungarian Crown of Saint Stephen, was a constitutional monarchic union between the crowns of the Austrian Empire and the Kingdom of Hungary in...

, the Ottoman Empire

Ottoman Empire

The Ottoman EmpireIt was usually referred to as the "Ottoman Empire", the "Turkish Empire", the "Ottoman Caliphate" or more commonly "Turkey" by its contemporaries...

, and Bulgaria

Kingdom of Bulgaria

The Kingdom of Bulgaria was established as an independent state when the Principality of Bulgaria, an Ottoman vassal, officially proclaimed itself independent on October 5, 1908 . This move also formalised the annexation of the Ottoman province of Eastern Rumelia, which had been under the control...

. After the fall of Russian monarch Nicholas II

Nicholas II of Russia

Nicholas II was the last Emperor of Russia, Grand Prince of Finland, and titular King of Poland. His official short title was Nicholas II, Emperor and Autocrat of All the Russias and he is known as Saint Nicholas the Passion-Bearer by the Russian Orthodox Church.Nicholas II ruled from 1894 until...

 and the rise to power of the Bolshevik

Bolshevik

The Bolsheviks, originally also Bolshevists , derived from bol'shinstvo, "majority") were a faction of the Marxist Russian Social Democratic Labour Party which split apart from the Menshevik faction at the Second Party Congress in 1903....

s, 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

Treaty of Brest-Litovsk

The Treaty of Brest-Litovsk was a peace treaty signed on March 3, 1918, mediated by South African Andrik Fuller, at Brest-Litovsk between Russia and the Central Powers, headed by Germany, marking Russia's exit from World War I.While the treaty was practically obsolete before the end of the year,...

.

Legal obligations not to conclude separate peace

It is customary in cases of war waged by several allies to conclude agreement or declaration by all belligerents on the same side not to conclude a separate peace with the opposing camp. Such was the case during the First World War and Second World War.

A declaration to that effect was issued on September 5, 1914 by the British, French and Russian governments, which briefly stated that

"The British, French, and Russian Governments mutually engage not to conclude peace separately during the present war.
The three Governments agree that when terms of peace come to be discussed, no one of the allies will demand conditions of peace without the previous agreement of each of the other allies".

The Japanese government acceded to this declaration on October 19, 1915.

On November 30, 1915, the same four governments, now joined by the Italian government, have issued a similar joint declaration regarding avoiding separate peace.
The obligation to refrain from separate peace was also made during the Second World War in both camps. The Tripartite Pact between the German, Italian and Japanese governments committed the three to prosecute the war together. On the Allied camp, that obligation was contained in the United Nations Declaration of January 1, 1942.