The first stanza of this poem lists events in nature - thunder, earthquake, lightning - that are heard by the world, that could cause great destruction and upheaval, and that could certainly awaken and frighten someone who is sleeping. However, the person being addressed in the poem is reassured that the turmoil will not disturb his rest.
The second stanza carries images of preparing to go to war. Soldiers drill in training, the drum and timbal (a type of drum) beat to call men to the fight, the fife "screams for blood." Again, the poet assures the hearer of this poem that s/he has "bled your best" and does not need to become involved in the impending fighting.
In the third stanza, an update on the fighting that has commenced is given; it's not going well. Those involved in the war must "charge to fall and swim to drown," but the audience is not in danger. As a soldier already dead and buried, the fallen fighter is safe with his "cloak of earth" protecting him, at peace and unaffected any more by the ongoing turmoil.