“Modern” can mean post-Romantic, and this poem (1867) does not fit the “Lyrical Ballads” definition of Romantic poetry, nor does it conform to traditional Victorian rules.  First, while it does mention Nature (“The sea is calm tonight”, etc.), Nature is not depicted as ideal or lifting.  The sea brings “the eternal note of sadness in.”  Even the classical reference to Sophocles tells of “the turbid ebb and flow /Of human misery.”  The next “modern” theme to be expressed is the absence of a Sea of Faith except for its “melancholy, long, withdrawing roar.”  This pessimistic strain is modern.  Finally, very modern, ie. unromantic,is the plea to “be true to one another” because “the world …hath really neither joy, nor love, nor light,/Nor certitude, nor peace nor help for pain;”  Compare this tone to any Romantic poem and the term Modern seems quite appropriate.  In terms of the mechanics of the poem, the rhyme scheme is there but obtuse; the lines do not scan in any simplistic way, and the stanzas are uneven – all departures from earlier poetic “rules.”  Finally, the depressing tone of the final line,“Where ignorant armies clash by night” is a signal that poetry is entering a modern attitude toward the unromantic destruction of warfare.