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It's kinda unavoidable that if one major power loses influence, another will benefit from the vacuum. You can't really oppose your own country's imperialism without making the case that other countries taking advantage is an acceptable risk.

This is more or less the story of WWI. With the increasing tensions and military buildup, socialists of countries across Europe formed the Second International and agreed in the Basel Declaration, which said that they would use the crisis to rise up simultaneously against every imperialist power and put an end to both the war and to capitalism:

If a war threatens to break out, it is the duty of the working classes and their parliamentary representatives in the countries involved supported by the coordinating activity of the International Socialist Bureau to exert every effort in order to prevent the outbreak of war by the means they consider most effective, which naturally vary according to the sharpening of the class struggle and the sharpening of the general political situation.

In case war should break out anyway it is their duty to intervene in favor of its speedy termination and with all their powers to utilize the economic and political crisis created by the war to arouse the people and thereby to hasten the downfall of capitalist class rule.

But once the war actually broke out, most of them found reasons to rally around their country's flag. German socialists pointed to the conditions of serfdom under the Tsar and pointed to the massive colonial empires of Britain and France, while British and French socialists argued that Germany undemocratic under the Kaiser and had more responsibility for starting the war. They mostly agreed that both sides were bad, but they said they were only fighting to safeguard their countries "against defeat" rather than for victory, but regardless, for all intents and purposes it was the same thing. Of course, in all of these countries, there was considerable political pressure and propaganda pushing them to fall in line and to regard the enemy as worse, and many people did what was personally advantageous regardless of what they had said previously.

There was only one exception, where the socialists took advantage of the war to overthrow their government, without regard for the possibility that it could help the other side, and they did end up ceding a fair bit of land too, but they were able to put a stop that that theater of the meat grinder everyone was being fed into.