From "John Brown's Body" by Stephen Vincent Benét, via a MT forum link in Mark Horstmann's "Things I Think I Think" mailing list. The complete poem can be found here. Mark, meanwhile, was alluding to the poem because of this New Yorker cartoon.
If you take a flat map And move wooden blocks upon it strategically, The thing looks well, the blocks behave as they should. The science of war is moving live men like blocks. And getting the blocks into place at a fixed moment. But it takes time to mold your men into blocks And flat maps turn into country where creeks and gullies Hamper your wooden squares. They stick in the brush, They are tired and rest, they straggle after ripe blackberries, And you cannot lift them up in your hand and move them. — A string of blocks curling smoothly around the left Of another string of blocks and crunching it up — It is all so clear in the maps, so clear in the mind, But the orders are slow, the men in the blocks are slow To move, when they start they take too long on the way — The General loses his stars, and the block-men die In unstrategic defiance of martial law Because still used to just being men, not block-parts.
And yet another fragment of the poem dealing with wooden blocks, more grim and more determined.