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Belarus itself probably would not be involved, it would be like last time, Russia would invade through Belarus.
The Russians have large numbers of soldiers training there which is why people think this. But I don't know how many they have and if that is enough.
Also, it's quite likely they really are just there for training. Russia mobilised a lot of men in a very short time, they probably need all the training infrastructure they can get and bases in Belarus have the advantage of being close to Ukraine as well.