1.Although Napoleon’s army entered Russia with far more supplies than they had in their previous campaigns, it had provisions for only twenty-four days.
Although Napoleon’s army entered Russia with far more supplies than (it had)for any previous campaign, it had provisions for only twenty-four days.
remember the last it suggest that the Napoleon's army should be a single noun, it.
and for many any previous campaign is parallal with for only twenty-four days.
because the sentence clear suggest that Napoleon’s army is the subject , omit it will not change the clear means. The omit after than is usual if compare with two定语with the same subject.