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As current 40k Space Marines are structured to small pitched battles and raids, I don't believe a single chapter could conquer Earth as we are now. But if we're to face a full planetary invasion from the Astra Militum, then I believe that the IoM would win eventually, through sheer attrition.
Would Earth unify in a common cause to fight back? I'm not sure, as modern armed forces and the world in general rely so much on satellite communications, the first casualty of the war would be the ability to talk to each other, as our satellites would most certainly be destroyed first, rendering our units incapable of receiving orders from higher echelons.