After the abdication of Nicholas II during the February Revolution and the subsequent Russian Provisional Government's overthrow by the Bolsheviks during the October Revolution, Russia defaulted on its commitments to the Triple Entente by signing a separate peace with Germany and its allies in 1917.
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).
The Tripartite Pact from September 27, 1940 between the German, Italian and Japanese governments committed the three to prosecute the war together.
A similar obligation arose within the Arab League, in the context of the Arab-Israeli conflict, not to reach any separate peace treaty with the Israeli government, in order to assure that a collective arrangement would take into consideration the interests of all Arab states plus the Palestinians.
The Egyptian government under Anwar Sadat acted in contrast to that rule when it decided to conclude a separate peace treaty in 1979.