In just three short lines, this poem sets two kinds of work against each other: fighting and writing. The title borrows the words of a well-known hymn, "Onward Christian soldiers," which calls believers to march like an army. The poem then quietly steps away from that call. It's the Lord's day, a day of rest, and the speaker spends it mining with a pen instead of marching anywhere.
The word "mine" is the clever turn. Mining suggests digging, patience, and searching for something valuable under the surface, and the speaker applies that to writing. It's also a word with a military ring, since mines are weapons, so the poem plays with the idea of war while rejecting it. The final line, "no time for war," sounds calm and a little dry, as if the speaker simply has better things to do.
The mood is peaceful and quietly defiant. There's no anger here, only a gentle choice: the speaker answers a battle hymn by sitting down to write.