In his concluding lines the speaker of "To Lucasta, Going to the Wars" talks about his love of honor. In fact, it's that very principle that ultimately gives meaning to his love for Lucasta, and his life more generally. While he only mentions honor one time, the preceding lines that justify his actions are all leading up to this big, principled reveal.
Honor is only mentioned in the last line of the poem, which suggests that often our principles influence our decisions without our even realizing that they do.
This speaker's not principled at all. He's just using honor as an excuse.