Fuller, Margaret. The Letters of Margaret Fuller. Edited by Robert N. Hudspeth. Vol. V. Ithaca: Cornell University Press, 1988.
Inferred Hudspeth identified: Fuller quotes Wordsworth's 'Character of the Happy Warrior' (first published in Poems in Two Volumes, 1807).
The specific lines quoted are: '...and sees what he foresaw.' Hudspeth provides the full stanza context (The Poetical Works of William Wordsworth, ed. Ernest de Selincourt and Helen Darbishire, 5 vols. [Oxford, 1940-49], 4:87): 'And, through the heat of conflict, keeps the law / In calmness made, and sees what he foresaw.'