4th verse of "The Star-Spangled Banner":
O thus be it ever, when freemen shall stand
Between their loved home and the war’s desolation.
Blest with vict’ry and peace, may the Heav’n rescued land
Praise the Power that hath made and preserved us a nation!
Then conquer we must, when our cause it is just,
And this be our motto: “In God is our trust.”
And the star-spangled banner in triumph shall wave
O’er the land of the free and the home of the brave
1' How to understand the "shall" in the first line?
Subjunctive mood? Just like the "be" in "it be ever thus" ("thus be it ever").
If so, as regards to the subjunctive mood, what's the difference between "the base form" and "modal verb + infinitive", especially in this context?
2' How to understand the "it" in the fifth line?
According to https://worldwar2collection.com/4th-verse-star-spangle-banner-the-meaning-behind-the-words/, the meaning of the line is "conquest is only necessary when there are just reasons to justify it."
Then why the line isn't "when our cause is just"? Why is there a burdensome "it"?
Since there is "it", shouldn't the line be equal to "Then we must conquer, when it is just our cause"? The "it" refers to the "we must conquer". The "just" is an adverb (meaning "exactly, precisely, simply"), instead of an adjective (meaning "righteous, honorable").
My instinct tells me "conquest is only necessary when there are just reasons to justify it" is the right answer, but I failed to analyze the line grammatically.
Update: So the "shall" in "freemen shall stand ..." is the same as in "the star-spangled banner in triumph shall wave" (the seventh line)?
related question: Three questions on "In full glory reflected now shines in the stream"