The title points to Macbeth, where a forest seems to march on a castle because soldiers carry branches as camouflage. The poem takes that image and asks what separates a wood from an army. Its answer is surprising: "love." A wood is alive and rooted, while an army is a force gathered to destroy.
The speaker seems wary of violence and of the people who stand by while it happens. "They that laugh and do nothing / only respect the grave" reads as a bitter jab at those who stay passive until death arrives. The next lines describe devastation, with strongholds razed and fields turned into sites of massacre.
The final lines turn personal. The speaker writes about men who do not "burn like candles," which suggests they don't give themselves up to be a light for others. The mood is somber and accusing, though the opening line leaves a thread of tenderness in the poem.