what began to unite the different opposition groups, such as monarchists, tsarist officers, former members of the nobility, and even liberal supporters of the provisional government, into a force that began to fight against the bolsheviks actively was

The correct answer to this open question is the following.

Although the question does not provide any options, we can answer the following.

What began to unite the different opposition groups, such as monarchists, tsarist officers, former members of the nobility, and even liberal supporters of the provisional government, into a force that began to fight against the bolsheviks actively was the Treaty of Brest-Litovsk.

Those were the times on many conflicts and changes in Russia. This treaty was signed on March 18, 1918. The participants of the peace agreement were the recent government of the Bolsheviks who had just accessed power in Russia and other countries such as Germany, the Ottomans, Bulgaria, and the Austria-Hungary Empire.  

However, this agreement ended with the allied victory in World War I, and the signing of the Treaty of Versailles.

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