The term 'Defencist Bloc' was used by V.I. Lenin and others to refer to the coalition of Mensheviks and Socialist-Revolutionaries who, after the February Revolution of 1917, dominated the soviets, and who wished to continue Russia's war effort. The term was applied to a variety of different groups, from those on the right who had been in favour of the war effort since 1914 and even supported aggressive war aims like territorial expansion, to those on the left who had adhered to the Zimmerwald anti-war movement before 1917 but approved of limited defensive war 'in defence of the revolution'. Since Defencists of these various types were often at odds with each other, the term 'Bloc' is misleading; however, the practical consequence of all varieties of Defencism under the February republic was that Russia remained in the First World War.
For further details and sources, please see Internationalist/Defencist Schism.