Corrected: "Either way will be difficult, for the President will want the Secretary of War to be inferior in rank to the commanding general, because then the former will be more of a personal appointee, and he himself will so gain a better control over the army."
The first "he" refers to the Secretary of War, the second to the President; the shift in antecedent produces complete obscurity.
No doubt Banquo was ambitious, but it did not master him as it did Macbeth.
Corrected: No doubt Banquo was ambitious, but ambition (or the craving for advancement) did not master him as it did Macbeth.
In the original form, "it" is without an antecedent, because a pronoun may not refer grammatically to an adjective. The reader is left to supply the noun "ambition" from the use of the adjective "ambitious."
We went to the Capitol determined to vote against him, but got there too late for our votes to count, which seemed to our adherents a most unwise policy.
Corrected: We went to the Capital determined to vote against him, though this determination seemed to our adherents a most unwise policy, but we got there too late for our votes to count.
"Which" is here ambiguous, because its antecedent is vague. Speaking strictly, it has no antecedent, for "which" may not refer grammatically to a phrase or a clause, but, like all pronouns, must have one word as its antecedent. Disregarding, however, the grammatical fault, we may still condemn the sentence, because "which" may mean either the determination to cast a negative vote or the carelessness of coming late.
1. They (the Danes) did not care for grammar, and, consequently, their influence unsettled the inflections of the language and prepared the way for their complete disappearance.
Corrected. They (the Danes) did not observe the rules of grammar, and, consequently, their influence unsettled the language and prepared the way for the complete disappearance of inflection.